docs: update Windows info

Add a section on worktrees to avoid symlink problems, and
note that Python 3 is now a hard requirement.

Change-Id: I83811db88692127c40cec8270f6f9486c639dc3f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256314
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>