Add documentation publishing workflow to standalone repo

[charliermarsh]

Summary

This approach is based on https://github.com/PrefectHQ/docs. Rather than publishing docs in the uv repo, we push to an independent repo that's used solely to house the docs. In Prefect's case, this allows them to publish versioned documentation (we should do that too). For us, though, the benefit is that we can publish the Ruff and uv docs as a single site (docs.astral.sh).

Prefect clones the source repo and builds the documentation from the docs repo (i.e., the action runs in the docs repo). In our case, I've instead set it up such that the action runs in uv (and later in ruff too), clones the docs repo, and puts up a PR in that separate repo. Because of these requirements, we have to use a PAT rather than a deploy key (as PATs cannot do GitHub-specific things like create PRs -- they can only operate over the Git CLI).

See: astral-sh/docs#2.

[charliermarsh]

This will take me 1,000 attempts to get working.

[charliermarsh]

Thanks @zanieb!

[zanieb]

Awesome :)