[repo config] Right sidebar metadata broken docs URL HTTP 404

[webknjaz]

It shouldn't be https://pyproject-api.readthedocs.io/en/latest/, but something like https://pyproject-api.rtfd.io. Fixable via repo settings by the maintainer only.

[gaborbernat]

Updated the repository homepage to https://pyproject-api.readthedocs.io, which resolves to the live docs. The README links were fixed in #291.