Apparently some Coq web servers are down, e.g. the manual and the Coq opam repo. INRIA is working on it, but it might take a bit.
For the manual the master branch version is still available here (https://coq.github.io/doc/master/refman/).
If you desperately needs to setup a fresh opam (say via Coq Platform) you can use the git repo (https://github.com/coq/opam-coq-archive). If you need instructions for this please ping me. I would have to figure out the exact details myself, but essentially in Coq Platform one would have to change the URLs here (https://github.com/coq/platform/blob/bb768b8431c0fd47a5f10b46cebce0105fd3fe4a/shell_scripts/install_opam.sh#L194)
You can also get a released version of the manual like this: https://coq.github.io/doc/V8.16.1/refman/
Last updated: Jan 29 2023 at 05:03 UTC