* Add links to the sidebar.
* Remove unnecessary `page-ref`s (these links are now in the sidebar).
* Small grammar/style fix.
* Fix code tabs.
* Remove {% page-ref %} uses.
* Fix some line lengths.
* Split out `custom.css` into multiple files.
* Remove duplicate titles.
* Shortens line lengths (to ~100 characters max per line).
* A few stylistic improvements
* Corrects a couple of typos
* Fixes a few grammar errors
Co-authored-by: Justin Starry <justin.starry@icloud.com>