doc/dev: first pass at more detailed CI docs

Created by: bobheadxi

Introduce introductory CI documentation for what's available in the generator and how to interface with it. Right now, the goal is to give a starting point for understanding where the touch points are within the generator to help get started with making changes, and not to document all of its features (e.g. each run type).

I also reorganized the dev docs homepage because it was very strange to get all these super detailed guides up-front when 90% of the time I want to find an overview first (in this case, the CI docs page).

Closes https://github.com/sourcegraph/sourcegraph/issues/25202