Move docs generation to main repository

[HonkingGoose]

Describe the proposed change(s).

@viceice wants to move the build stuff from the docs repository into the main repository. Publish still needs to be on the docs repository.

Describe why we need/want these change(s).

I tried the new MkDocs 1.5 link checking feature [^1], with their recommended strictness checks. It reports 460 warnings for links in the docs:

• https://github.com/renovatebot/renovatebot.github.io/pull/305

We should fix the problems, and thus make our links more robust. I think fixing the links is easier when the source files are in the main repository?

Related issue:

• https://github.com/renovatebot/renovatebot.github.io/issues/304

[^1]: MkDocs, validation

[viceice]

publish needs still be on the docs repo, but all build stuff will be moved here.

[HonkingGoose]

@viceice Checking in to see if any of our recent changes affect this issue. We're moving two files into the main repository:

• Homepage for the docs: index.md file
• The release notes

[viceice]

Need to check what needs to be done to close this