Update discord link in docs

[jolheiser]

I've been meaning to "sneak" this change in with other docs changes for a while, but I keep forgetting.
So here's a PR solely to update the Discord links. 😄

[codecov-io]

Codecov Report

Merging #10263 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10263      +/-   ##
==========================================
+ Coverage   43.65%   43.67%   +0.01%     
==========================================
  Files         584      584              
  Lines       80954    80954              
==========================================
+ Hits        35344    35356      +12     
+ Misses      41229    41219      -10     
+ Partials     4381     4379       -2

Impacted Files	Coverage Δ
modules/queue/unique_queue_disk_channel.go	52.56% <0%> (-1.93%)	⬇️
services/pull/check.go	35.36% <0%> (-0.61%)	⬇️
services/pull/patch.go	62.89% <0%> (+2.51%)	⬆️
services/pull/temp_repo.go	31.62% <0%> (+2.56%)	⬆️
modules/git/repo_language_stats.go	68.91% <0%> (+4.05%)	⬆️
modules/indexer/stats/db.go	68.75% <0%> (+18.75%)	⬆️

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