docs: Add conda-forge as unofficial install option
Description
As described in #7946 (closed) by @matthewfeickert, add the unofficial conda-forge install instructions.
I did a couple of extra things while I was here, because I couldn't pretend I didn't see them:
- Alphabetical order for the Linux install options. Shockingly, technical writers love their alphabetical order.
- Adding a parenthetical to the homebrew option in the linux section of the table of contents, to make clearer it's the supported option.
I think we can safely close #7946 (closed) after this, but I'll want a spot check from someone on the team.
Related issues and merge requests
Related to Note in install docs that glab exists on conda-... (#7946 - closed)