chore(slack): Added New commands to /sentry help

[iamrajjoshi]

After we added bot commands for support and docs, we should add them to the list of commands that we show the user when they ask for help via /sentry help

Addresses bot-7

[cathteng]

we should reorganize the text, some of the commands can be run in both a channel and a DM, only the link/unlink team commands can only be run in a channel

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.15%. Comparing base (02217e1) to head (eadd493).
Report is 48 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #74017       +/-   ##
===========================================
+ Coverage   54.30%   78.15%   +23.84%     
===========================================
  Files        5598     6651     +1053     
  Lines      236100   297366    +61266     
  Branches    40459    51207    +10748     
===========================================
+ Hits       128206   232393   +104187     
+ Misses     106485    58705    -47780     
- Partials     1409     6268     +4859     

Files	Coverage Δ
.../sentry/integrations/slack/message_builder/help.py	85.41% <100.00%> (+21.78%)	⬆️

... and 2314 files with indirect coverage changes