Issues with Release Guide

*Description*:
There are several issues that I ran into while doing the release of 0.4.0 following the release guide in our docs:

- [x] 1. We need to add a step to update the compatibility matrix since I forgot this https://github.com/envoyproxy/gateway/pull/1356

- [ ] 2. (optional) We should add a step to remember to update `docs/index.html`, `docs/v{MAJOR}.{MINOR}.{PATCH}/releases.rst` and `docs/latest/releases.rst`.
- [ ] 3. The release guide doesn't instruct you to PR docs updates into the release branch. This is a problem since if you only PR the docs updated into `master` then the tag you cut from the release branch will be missing the docs and CI requires these files in the pushed release tag.
- [ ] 4. The release guide instructs you to `make docs-release TAG=v{MAJOR}.{MINOR}` which breaks from the current pattern of `make docs-release TAG=v{MAJOR}.{MINOR}.{PATCH}`. This causes problems such as all of the docs that are generated containing instructions to use the wrong docker image tag and helm version (ex: `v0.4` instead of `v0.4.0`).
- For this point in particular I think a separate issue is warranted about how we want to handle this from now on. See https://github.com/envoyproxy/gateway/issues/1358 for details on the decision about how we should handle docs directories for future releases.