docs: add Gerrit to list of supported platforms

[HonkingGoose]

Changes:

• Add Gerrit to list of supported platforms, and link to the Gerrit platform docs for Renovate
• Mark the platform (experimental)

Context:

This upstream PR must be merged first:

• https://github.com/renovatebot/renovate/pull/18961

[HonkingGoose]

Yay! The mkdocs build in strict mode is failing! :partying_face:

 pipenv run mkdocs build
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /home/runner/work/renovatebot.github.io/renovatebot.github.io/site
WARNING -  Doc file 'index.md' contains a relative link 'modules/platform/gerrit/index.md', but the target is not found among documentation files.

Aborted with 1 warnings in strict mode!
make: *** [Makefile:14: build-docs] Error 1
Error: Process completed with exit code 2.

This is correct, because that source file is missing. The file must be created upstream first:

• https://github.com/renovatebot/renovate/pull/18961

So this stops us from accidentally merging my PR before it's ready. :smile:

[HonkingGoose]

I'm putting this back into draft. The mkdocs build step passes, but the new Gerrit link on the homepage leads to a "empty page".

Probably related to a problem with the git submodule update here, which is currently pointing to commit 1757742 on the main Renovate repository:

• https://github.com/renovatebot/renovatebot.github.io/pull/384 (includes the change from https://github.com/renovatebot/renovate/pull/26479)