APIv3 docs and Pagure integration docs

[Michal Novotny]

Hello!

as we promised in the last release notification email, here is
documentation for APIv3:

https://python-copr.readthedocs.io/en/latest/ClientV3.html

and here for Pagure integration:

https://docs.pagure.org/copr.copr/user_documentation.html#pagure-integration

Both of those new features are pretty great in our opinion so give it a
look if you have time.

Best regards
Copr team