There are quite a moments, when Coq main site https://coq.inria.fr/download is down. Has Coq any downloads available from its main github repository https://github.com/coq/coq or some other mirrors? AFAK, e.g. Isabelle has Cambridge and Munich sites.
I am working on my math pet project on the weekends almost exclusively, so, it would be sad to wait for the next week and hope that the site will be up.
Maybe https://github.com/coq/platform/releases has the binary downloads, but it has 8.16 only.
There are a number of alternatives.
The binary platform downloads suggested by @ice1000 are the easiest out of the box.
Incidently, the Coq 8.17 platform release was supposed to be out by now, but was delayed by the same coq.inria.fr outage you encountered.
If you want to install with opam (instructions can be found on the Web Archive version of the Coq website), another alternative, shared by Paolo Giarrusso on Zulip, is to "clone the [Coq opam] repo locally and use opam's support for local folders as repos":
git clone https://github.com/coq/opam-coq-archive.git
opam repo add --kind local coq-fork "$(pwd)/opam-coq-archive/released/"
and similarly for the dev versions if you'd like. This works because it's only the index that's down; most packages are hosted on GitHub rather than coq.inria.fr servers. This should work indefinitely, though you'll have to pull in the git repo manually to update, in addition to opam update, and Karl Palmskog said on Zulip that git pull && opam update might be a bit slower this way.
The Web Archive version of the Coq website suggests a number of other installation methods: nix, snap, and docker images included.
You can also use jsCoq directly in your browser, though it might only support single-file projects, I'm not sure.
