docs: remove sourcegraph.css in favor of bootstrap
Created by: ryan-blunden
The reliance on sourcegraph.css brought a lot of un-used styles. This a step in the right direction, using bootstrap 4.3.1 and extracting the variables and styles from sourcegraph.css to ensure no (or very little) visual differences.
Also:
- added the footer from the new about site.
- Fixed a bug where opening menus in tablet and mobile would open the version selector.
- Formatted code.