Extensions discoverability

Created by: christinaforney

The web team will be doing a first effort to improve the extension registry in the next week or so (first week of July): https://github.com/sourcegraph/sourcegraph/issues/11743

Add to main navigation?
Improve extension search and discovery
- Link to docs on how to create extension
- Replace category dropdown with tags
- Replace enabled/disabled drop down with checkbox
- Add lead in for authoring extensions