Document asdf for project and refactor "prepare your system"

What does this Merge Request do?

The package manager asdf is supported by gitlab-development-kit now: !1330 (merged), !1436 (merged).

With that in place, and with a view to making the instructions simpler by using as much common documentation as possible (asdf instructions are the same for Linux and macOS), this MR:

• Documents asdf for the first time.
• Reorganizes https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/prepare.md, so that "base" dependencies are handled first, then asdf dependencies after.
• Completely refactors the macOS instructions, as they were easy for me to validate. They are also a common case at GitLab (particularly non-engineering).
• Removes MacPorts instructions, I think brew instructions are enough.
• Puts a note against Linux instructions that they aren't completely up-to-date. I didn't want to:
  • Make the diff on this MR any bigger.
  • Block merging this MR by needing to test the Linux instructions.

It would be good after merging this to continually refine the page by doing clean installs of GDK on all the Linux distributions we document (doing as much in asdf as possible). If we can't validate some of the instructions in short order, we might remove some instructions and allow them to be added back by interested folks at a later date.

• Closes: #516 (closed), an issue to document asdf (though this MR gives asdf primacy, rather than leaving rbenv and rvm as co-equal options)
• Closes: #911 (closed), which called for a reorganization of prepare.md. This MR moves us toward being able to "prepare a system" in as few steps as possible.
• Closes: #477 (closed), which was raised because the prepare.md file was difficult to follow.

CC @mikelewis @deuley @toon @tigerwnz @rpaik @rdickenson @ashmckenzie

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.