add doc for dev feature flags

[christianvogt]

Adds a small blurb to our documents about dev feature flags and how to enable them in the browser using ?devFeatureFlags query string.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.32%. Comparing base (3a76f55) to head (cc07e04).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3211      +/-   ##
==========================================
+ Coverage   85.30%   85.32%   +0.02%     
==========================================
  Files        1267     1268       +1     
  Lines       27833    27868      +35     
  Branches     7411     7415       +4     
==========================================
+ Hits        23742    23778      +36     
+ Misses       4091     4090       -1     

see 15 files with indirect coverage changes

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[openshift-ci]

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