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`.)