Add bors to CI, or activate GH merge queue

[jyn514]

This would have prevented master being red earlier this morning (#1049).

Steps to implement:

• Add the homu config (example)
• Add the permission on the team repo (example)
• Add a "bors build finished" CI job. Relevant code: https://github.com/rust-lang/crater/blob/dcc54566d5ec82cabca435c0145cbd06f32d3602/.github/workflows/bors.yml#L126-L150

The repo should be called docs-rs on the bors side.

[pietroalbini]

New instructions: https://forge.rust-lang.org/infra/docs/bors.html#adding-a-new-repository-to-bors

[syphar]

Currently we don't have that many PRs merged, so this doesn't really happen regularly.