De-emphasize mailing list in landing page

[abyrd]

My understanding is that a decision was made to use the mailing list only for announcements, and the Gitter chat for all other support and discussion. We are still getting questions posted to the mailing list, which then requires someone to reply encouraging the person to use Gitter, which means the whole list will receive these questions and response emails. I went looking for why this was happening, and see that the landing page of the docs still says "The fastest way to get help is to use our Gitter chat room where most of the core developers are. You can also send questions and comments to the mailing list...". This PR changes the part about the mailing list to say it's used only for announcements.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.88%. Comparing base (e2cc0e5) to head (3572d3b).

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5820      +/-   ##
=============================================
- Coverage      67.88%   67.88%   -0.01%     
- Complexity     16557    16558       +1     
=============================================
  Files           1908     1908              
  Lines          72355    72355              
  Branches        7443     7443              
=============================================
- Hits           49119    49117       -2     
- Misses         20714    20716       +2     
  Partials        2522     2522              

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