* Drop mkdocs from `requirements.txt` and add Zensical
* Replace mkdocs with Zensical in CI and pre-commit tasks
* Remove `.info` from the `docs/` build directory (obsolete)
* Update the legacy ReadTheDocs configuration
* Update upgrade script to use Zensical
* Remove custom docs footer
* Remove obsolete CSS
* Add a background-color to img elements in docs to ensure readability in dark mode
* Limit style changes to those within CMS content blocks; update colors of main netbox_logo.svg
* Add a white stroke to the main logo
* Add light & dark mode versions of the NetBox logo
---------
Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>