docs: advertise the Discord server

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).

Summarize your changes:

Does what it says on the tin.

[codecov]

Codecov Report

Merging #2181 (4d0a3b4) into master (4a679ac) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2181   +/-   ##
=======================================
  Coverage   72.66%   72.66%           
=======================================
  Files          74       74           
  Lines        2221     2221           
  Branches      420      420           
=======================================
  Hits         1614     1614           
  Misses        448      448           
  Partials      159      159           

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