Disable suggesting reviewers in docs repository

[wouterj]

I shortly discussed this with the doc team, and I think this feature is not applicable to the docs repo:

• It's very common for us to not respond to a PR within 20 hours
• Many documents are created by us, the community mostly fixes small stuff. Also, 1 document contains many features, editing a line somewhere in the document doesn't indicate you're the expert for other features in the document. E.g. in Update best_practices.rst symfony/symfony-docs#14850, the pinged user modified the example README: symfony/symfony-docs@6e8d393