Add pages for GH repositories to help in transition

[hobofan]

I think it would be nice to have pages like /gh/tweag/rules_nixpkgs (/gh/GH_OWNER/GH_REPO) for all the modules and repositories from the SIG catalog.

The main goal of this would be to help the Bazel users in transitioning to bzlmod. It would likely only be useful for the transition phase (next 1-2 years), but could make it a lot easier for users to do the transition.

On that page we could either:

• display generic instructions on how to switch to the bzlmod module for the repository, if one has been submitted to the BCR
• display a link to specific migration notes (if provided by the repository)
• display a link to a tracking issue in the repository that outlines the current status for bzlmod support

With those things in place, it would be a lot easier to find all the required information for transferring from WORKSPACE file to bzlmod. I could also imagine that the same data used for this could be used to write a small CLI tool that could help people in transitioning their projects (if someone finds themselves with too much energy 🙃).

[hobofan]

As discussed in a previous SIG meeting, this won't be added to the bcr-ui, as that will stay focused on BCR-contained information only.

Instead this may become part of a separate site which incorporates data from the SIG catalog.

Keeping this open until we start that separate site, and will move the issue there then.