Channel branches are now provided in the nixpkgs repo.

Issue #71176.

This makes things more consistent. It’s also how GitHub refers to pull
requests.

The server is not verified over the git:// transfer protocol. If you
clone a repository over git://, you should check if the latest commit's
hash is correct.

On the other hand, https:// will always verify the server automatically,
using certificate authorities.

They redirect to HTTPS anyway. This also contains some minor content
changes.

This information isn't useful anymore since we disabled all Travis CI
builds in 44917c46.

* readme.md: reincarnate wiki

It is kinda strange to announce that wiki is deprecated, while it is in fact growing.
I'd say, the "Move-the-wiki" milestone is stalled instead.

* Readme.md: rename to community maintained wiki

This makes them look consistent with the travis badge.

We probably no longer want to advertise it.

Turns out it was already up to date and I just had a stale cache!

Seems like the community was in support of changing it, and someone went and changed it on the homepage, so we might as well stay consistent.

@vcunat dropped `Main_Page` suffix from the link.