Allow customizing sidebar

[vsalvino]

• Move sidebar toc into globaltoc template
• Allow customizing sidebars per usual sphinx manner.
• Add customizing docs.
• Removed undocumented is_homepage theme option (which seemingly only disabled the search box?)

See: https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_sidebars

[vsalvino]

To help test, I have created a build of a custom project using all my open PRs merged together: https://vince-temp.codered.cloud/

[Scotchester]

Good stuff, thanks!