updated links to discussion forum

[orbeckst]

• use new GitHub discussions instead of old mailing list
• also link to MDAnalysis community resources for discord

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.70%. Comparing base (f04025d) to head (68325b2).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #146   +/-   ##
=======================================
  Coverage   87.70%   87.70%           
=======================================
  Files           5        5           
  Lines         748      748           
  Branches       97       97           
=======================================
  Hits          656      656           
  Misses         56       56           
  Partials       36       36           

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[orbeckst]

This seemed uncontroversial so I just merged.