meta: move repo to Tangled #13

It would be cool if ATFile's primary source was Tangled: fitting since they're both ATProto-powered.

In its current Alpha stage, we need to take a few pitfalls into account. Tangled does not yet support:

• Pull Requests
• Releases
• HTML in Markdown files (@tangled.sh/core#28)

For this, we'll have to continue to use GitHub for such features. Beyond that, we also need to consider:

• GitHub is still the sourceforge that most people recognize, so it makes little sense to remove it from there entirely
• PRs cannot be disabled on GitHub, so we'll continue to accept PRs from there too

However, we can disable Issues on GitHub and ask people to create issues on Tangled. After all, someone using ATFile will already have an ATProto account: they can start using Tangled straight away too!

We'll also need to change any references in the README and source code referencing GitHub (except for Releases for now).