We're migrating to using the cleaner zulip.com domain, which involves
changing all of our links from ReadTheDocs and other places to point
to the cleaner URL.
This deserves a fact-check to make sure I haven't simplified
anything past what's true! Note in particular the TODO comments.
Especially in particular, I took out references to the .zip installer
for macOS and the .deb installer for Linux. I couldn't tell in what
situation we'd really want to steer a user toward them (and wouldn't
recommend instead either the .dmg installer, or the apt or AppImage
instructions, respectively), so taking them out simplifies the
instructions and avoids presenting the user with a choice they have no
context for making. But if there is such a situation, then we can put
them back.