docs: Add troubleshooting section on accessing LS data

[caitlinwheeless]

Adding a troubleshooting section that links to https://labelstud.io/guide/ml#Allow-the-ML-backend-to-access-Label-Studio-data.

Also clarified warning note

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.83%. Comparing base (31d0867) to head (1a2b123).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
+ Coverage   50.32%   50.83%   +0.51%     
==========================================
  Files          35       38       +3     
  Lines        2180     2526     +346     
==========================================
+ Hits         1097     1284     +187     
- Misses       1083     1242     +159     

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