Improve docs for code highlighting with line numbers (#974)

* 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.