Auto merge of #5373 - flip1995:doc_release_backport, r=Manishearth

Document how to create releases and backports

This PR adds documentation on how to create Clippy releases and backports.

This PR also introduces a new backport/release process for Clippy:

- A beta branch is introduced: https://github.com/rust-lang/rust-clippy/tree/beta
- Backports are now done by PRs to the `beta` branch
- also commits to the beta branch will be deployed, so that the documentation page has a tab for the Clippy beta release.

This has two advantages:

1. The Clippy release process is closer to the Rust release process: stable releases are tagged, the beta release has its own branch
2. Backports to beta are easier, since they don't need as much coordination as before. No new branch has to be created for a backport. Just a PR to the beta branch, like in the Rust repo.¹

I tested the deployment here: https://github.com/flip1995/rust-clippy/runs/534465081 and it created this commit: https://github.com/flip1995/rust-clippy/commit/734503377e5ade55864ee674c010227a780cbf34, so it works.

r? @Manishearth your thoughts?

[Rendered (release.md)](https://github.com/flip1995/rust-clippy/blob/doc_release_backport/doc/release.md)
[Rendered (backport.md)](https://github.com/flip1995/rust-clippy/blob/doc_release_backport/doc/backport.md)

changelog: none

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¹: For this, we may have to modify the CI, so that beta rustc is used to check PRs to beta (or we test it locally, and merge it without bors)

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