Close down the docs repo somewhat

[langner]

Since this repo is automatically updated, let's close down most of the unnecessary parts.

• Close all other existing issues.
• Don't allow new issues (in Settings).
• Add a README file to redirect people to the main repo - not sure if this needs to be integrated with the automated update somehow.
• Anything else?

@berquist any other concerns here?

[berquist]

I don't think so.

Add a README file to redirect people to the main repo - not sure if this needs to be integrated with the automated update somehow.

Yes. Once the README is written, it can be stored under doc and copied over by the deploy script.

[langner]

I think shutting down issues will make this issue invisible as well, so will do that at the end.

[berquist]

We discovered in cclib/cclib#1154 that after migrating from Travis CI to GitHub Actions, we never brought back automatic updating of this repository. That issue is tracking it.