Remove discord channel link and change the runtime link

[jchapuis]

No description provided.

[what-the-diff]

PR Summary

• Removal of the discord.md file
  This PR has eliminated the discord.md file. This might suggest that related content could have been migrated elsewhere or has become obsolete.

• Modification in index.md file
  In the context of the index.md file, the section previously known as "Akka runtime" has been renamed to "Runtimes". This could potentially result in more clarity or better organization within the file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aa1ba0f) 89.73% compared to head (c65673e) 89.61%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
- Coverage   89.73%   89.61%   -0.12%     
==========================================
  Files          73       73              
  Lines         877      867      -10     
  Branches       94       98       +4     
==========================================
- Hits          787      777      -10     
  Misses         90       90              

see 2 files with indirect coverage changes