Fix excluded vendor cleanup (#985)

Fix #973

Update the array of excluded directories and files to be:

- those that Jekyll excludes by default (since our `exclude` replaces Jekyll's defaults in Jekyll 3) and
- the others that the theme previously excluded, but omitting `vendor` – also ones that are no longer in the repo.

Excluding `vendor` was preventing HTML compression of the theme docs, since the layout used for compression is in `_layouts/vendor/compress.html`.

The change from one-line to multi-line YAML layout for the array of excluded files enhances readability. The addition of explanatory comments supports maintenance.

To test the effect of this PR on the theme docs:

- Build and serve the theme docs.
- Check that  the HTML files are now compressed.
- Check that the last example of code with line numbers has incorrect formatting.

(This PR cannot affect sites that use the theme as a gem or as a remote theme, since we don't include our `_config.yml`.)

_config.yml

@@ -19,7 +19,29 @@ baseurl: "/just-the-docs" # the subpath of your site, e.g. /blog
 url: "https://just-the-docs.github.io" # the base hostname & protocol for your site, e.g. http://example.com
 
 permalink: pretty
-exclude: ["node_modules/", "*.gemspec", "*.gem", "Gemfile", "Gemfile.lock", "package.json", "package-lock.json",  "script/", "LICENSE.txt", "lib/", "bin/", "README.md", "Rakefile", "vendor"]
+
+exclude:
+ # from https://github.com/jekyll/jekyll/blob/master/lib/site_template/_config.yml:
+   - .sass-cache/
+   - .jekyll-cache/
+   - gemfiles/
+   - Gemfile
+   - Gemfile.lock
+   - node_modules/
+   - vendor/bundle/
+   - vendor/cache/
+   - vendor/gems/
+   - vendor/ruby/
+ # specific to the theme website:
+   - bin/
+   - lib/
+   - "*.gemspec"
+   - "*.gem"
+   - LICENSE.txt
+   - package.json
+   - package-lock.json
+   - Rakefile
+   - README.md
 
 # Set a path/url to a logo that will be displayed instead of the title
 #logo: "/assets/images/just-the-docs.png"