docs(wandb): explain how to use W&B integration

[borisdayma]

Documentation on how to use W&B integration has been added to clear up confusion on how to customize logging.

fix #5262

[codecov]

Codecov Report

Merging #5607 into master will increase coverage by 1.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5607      +/-   ##
==========================================
+ Coverage   76.88%   78.11%   +1.23%     
==========================================
  Files         145      145              
  Lines       25351    25351              
==========================================
+ Hits        19491    19804     +313     
+ Misses       5860     5547     -313     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_roberta.py	43.98% <0.00%> (-49.38%)	⬇️
src/transformers/file_utils.py	79.26% <0.00%> (-0.34%)	⬇️
src/transformers/modeling_tf_mobilebert.py	96.77% <0.00%> (+73.38%)	⬆️

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