* Add toc heading custom include

Closes #961.

* Revert "Add toc heading custom include"

This reverts commit 49813c341973e313db0a21f075a60ebf2120989e.

* Update code highlighting with line numbers

- Add the example of code highlighting with line numbers explained in the [Jekyll docs](https://jekyllrb.com/docs/liquid/tags/#line-numbers).
- Fix the styling of narrow code with line numbers, which floats to the centre without this CSS adjustment. (The line numbers column expands as needed with larger numbers of lines, despite using `width`; using `min-width` doesn't work.)

To apply HTML compression, I removed `vendor` from `exclude`; that change is left to a different PR.

doc for turning on kramdown linenos globally corrected/

Optimised CSS selectors.
Changed markup in `linenos-test` to inhibit line numbers appearing in YAML examples when testing code fences.

The additions to `_config.yml` go together with the changes to `code.scss`,
to facilitate adjusttments by users. See `docs/linenos-test` for the details.
The CSS code has been significantly refactored and simplified,
and seems to produce sensible results (at least on Safari and Firefox).

This should revert the changes made to index-test.md