Add coverage areas docs page and refer from landing page

[zstumgoren]

Add a Coverage areas documentation page that explains in more detail which geographic areas and platforms we currently cover (at time of writing, only CivicPlus, with plans to add Legistar soon). We should prominently mention and link to these docs from the landing page/QuickStart section, though do so in a general way that doesn't require updating every time we add a new platform.