docs: Refactor/rewrite of "Get involved" page
    
    This commit is a significant rewrite of our "Get involved" page. I
    expanded some parts, rewrote some parts to be more clear, fixed a lot of
    URLs, and removed some parts that are not a major part of actually
    getting involved as a team member. My edits are briefly explained below:
    
    * Create FAS account: Fix links, try to better explain why this is
      important/necessary
    * Wiki homepage: Realistically the wiki homepage is not critical and
      there are new barriers to editing the wiki because of anti-spam
      measures (CLA+1 required)
    * Mailing list: Better explain how we use our mailing list
    * Self-introduction: Attempted to make questions feel more genuinely
      curious - they felt a little intimidating to me in retrospect!
    * Group chat: Properly explain IRC/Telegram, add links to join either
    * First steps: Fix `xref` to CommOps docs
    * Meetings: Better explain how our meetings work as of 2019
    * Pagure tickets: Clarify our workflow for 2019, update/fix URLs, and
      emphasize importance of new ideas into the mix too :)
    * FAS group: Drop this completely, it is not a major part of our
      everyday work and the group is invite-only now anyways (i.e. nobody
      can join the group on their own anymore)