Allow 404 nesting; clean up sidebar code

[zkamvar]

This will fix #498 by no longer relying on regular expressions to edit and modify the 404 HTML content.

This also does the following

• removes the needless recursion from update_sidebar()
• updates the documentation for sidebar functions and collects them into sidebar and sidebar_item.
• updates the documentation for build_html

[zkamvar]

I have tested this with the integration test suite and can confirm that it works: https://htmlpreview.github.io/?https://github.com/carpentries/workbench-integration-test/blob/5c6bd9262c0bebaf3d189a6b7cda39c3ec7eb1ac/404.html

As a bonus, it allowed me to find a bug that I was not testing for 😬