Turn this into a proper root page

[Sancus]

This repository should probably contain something like an automated index of all GitHub pages inside the Thunderbird org and a list of other resources/links. This can probably be created with the GitHub API and an Action.

Individual projects can have their own repositories and still publish to thunderbird.github.io/reponame, like how https://github.com/thunderbird/heatmap does.

None of this is urgent, however.