* Fix stylelint "declaration-block-no-redundant-longhand-properties"

Co-authored-by: Matt Wang <matt@matthewwang.me>

Context: #1074, #1076. I think the problem is likely with `@use "sass:math";`; the stock pages image doesn't contain an up-to-date enough version of SASS. I've instead replaced just that instance with a runtime `calc()` operation, which *should* get optimized away by the compiler (see: [SASS docs](https://sass-lang.com/documentation/breaking-changes/slash-div#transition-period)).

---

Original PR body:

> @pdmosses noticed that we have deprecation warnings on some of our SASS code. After testing locally, all of them have to do with using / as division in SASS, which is [deprecated](https://sass-lang.com/documentation/breaking-changes/slash-div) (since there's some lexical ambiguity).
> 
> SASS has a nifty [migrator tool](https://github.com/sass/migrator). I used the migrator piecewise on each deprecation warning (since the global usage fails on some liquid code). Upon manual inspection, I think there are no false positives. Running bundle exec jekyll serve after a fresh install and bundle update no longer emits any warnings.

@pdmosses noticed that we have deprecation warnings on some of our SASS code. After testing locally, all of them have to do with using `/` as division in SASS, which is [deprecated](https://sass-lang.com/documentation/breaking-changes/slash-div) (since there's some lexical ambiguity). 

SASS has a nifty [migrator tool](https://github.com/sass/migrator). I used the migrator piecewise on each deprecation warning (since the global usage fails on some liquid code). Upon manual inspection, I think there are no false positives. Running `bundle exec jekyll serve` after a fresh install and `bundle update` no longer emits any warnings.

Closes #1073; blocked by #1072 (CI failure).

This is a catch-all PR that modernizes and updates our Stylelint config, and resolves all open issues. This is a pretty big change - so I want to update all of our related dependencies in lockstep.

In particular, this PR

- [x] updates stylelint to `v14`
- [x] adds in the standard stylelint config for SCSS (`stylelint-config-standard-scss`)
- [x] swaps out `stylelint-config-prettier` for `stylelint-config-prettier-scss`
- [x] ~~properly update `@primer`-related plugins:~~ completely remove `primer` from our configuration
- [x] autofix, manually resolve, or disable all newly-introduced lint errors; **I've avoided manually resolving errors that would be a behavioural change**
- [x] re-runs `npm run format`

See the "next steps" section on some extra thoughts on disabling errors.

(implicitly, I'm also using node 16/the new package-lock format).

### disabling rules and next steps

I've introduced several new disabled rules. Let me quickly explain what's going on; there are two categories of rules I've disabled:

1. rules that were temporary disables; they were frequent enough that I couldn't manually resolve them, but should be simple. **I plan on opening issues to re-enable each of these rules**, just after this PR
    - `declaration-block-no-redundant-longhand-properties`: this is just tedious and error-prone
    - `no-descending-specificity`: this one is tricky since it could have impacts on the cascade (though that seems unlikely)
    - `scss/no-global-function-names`: I think we need to [import map and then use `map.get`](https://stackoverflow.com/questions/64210390/sass-map-get-doesnt-work-map-get-does-what-gives), but I'll leave this as out of scope for now
2. rules that are long-term disables; due to the SASS-based nature of our theme, I think we'll keep these in limbo
    - `alpha-value-notation` causes problems with SASS using the `modern` syntax - literals like `50%` are not properly interpolated, and they cause formatting issues on the site
    - `color-function-notation` also causes problems with SASS, but in this case the `modern` syntax breaks SASS compilation; we're not alone (see this [SO post](https://stackoverflow.com/questions/71805735/error-function-rgb-is-missing-argument-green-in-sass)). 

In addition, we have many inline `stylelint-disable` comments. I'd open a separate issue to audit them, especially since I think some disables are unnecessary.

### on Primer 

**note: there hasn't been much other discussion, so I'm going to remove primer's stylelint config.**

If I do add `@primer/stylelint-config`, I get *a ton* of errors about now using `@primer`'s in-built SCSS variables. I imagine that we probably won't want to use these presets (though I could be wrong). In that case, I think we could either:

1. disable all of those rules
4. not use `@primer/stylelint-config`, since we're not actually using primer, and shift back to the standard SCSS config provided by Stylelint

~~Any thoughts here? I also don't have the original context as to why we do use the primer rules, perhaps @pmarsceill can chime in?~~

Based on #238, resolves #66.

This PR combines (and resolves conflicts between) #448, #463, #466, #494, #495, #496, #498, and #572. 

The main aim is to facilitate use of several of the implemented features _together_, when using the fork as a remote theme. It should also simplify merging the included PRs into a future release.

The branch [combination-rec-nav](https://github.com/pdmosses/just-the-docs/tree/combination-rec-nav) adds [multi-level navigation](https://github.com/pmarsceill/just-the-docs/pull/462) and (NEW:) [sibling links](https://github.com/pmarsceill/just-the-docs/pull/394) to the branch used for this PR. It includes updated [documentation for the navigation structure](https://pdmosses.github.io/just-the-docs/docs/navigation-structure/), and reorganised and extended [navigation tests](https://pdmosses.github.io/just-the-docs/tests/navigation/). The documentation and the tests can be browsed at the (temporary) [website published from the combination-rec-nav branch](https://pdmosses.github.io/just-the-docs/

).

_Caveat:_ The changes to v0.3.3 in this PR and #462 have not yet been reviewed or approved, and may need updating before merging into a release of the theme. If you use a branch from a PR as a remote theme, there is a risk of such updates affecting your website. Moreover, these branches are likely to be deleted after they have been merged. To avoid  such problems, you could copy the branch that you want to use to your own fork of the theme.

Co-authored-by: Matt Wang <matt@matthewwang.me>

(cherry picked from commit 2d6e97e0cbfb8bdcf87a2e5ad19bd59fc8f5405d)

(cherry picked from commit 5dce2adc2871e6814dbf42a658e6b364f4dc6641)

(cherry picked from commit f25d76730b175fb680aa2770fb20a67fd858ffbb)

(cherry picked from commit 2a8901ff953004a2fc7cebde86bae6575cec2924)

(cherry picked from commit db50f282bab1e98397cc7434a811b8b7da2fe6dc)

(cherry picked from commit 6684f7c7faa15d6d59df35fbff9ea6666634d754)

(cherry picked from commit fd3c68cd1a39e059180c35c1d56ffa33ef4dfde4)

(cherry picked from commit 2b153ed074811e735aaa58d57a3fcdf7838e17f5)