chore: adding feature flags v1 documentation in Release Notes

[srinify]

SUMMARY

At the moment, the Apache Superset website is unreliably tapping into browser cache. This means that the documentation on 1.0 feature flags isn't accessible in a reliable way.

As we work on / resolve the ASF website caching issues (and find a longer term documentation habit for feature flags), I'm mirroring the documentation in the Release Notes for 1.0 so we can at least have a reliable way to share this information with the community!

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[villebro]

LGTM!

[codecov-io]

Codecov Report

Merging #12813 (fdbf49d) into master (32f2c45) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12813      +/-   ##
==========================================
- Coverage   67.01%   67.00%   -0.01%     
==========================================
  Files        1022     1022              
  Lines       50102    50102              
  Branches     5191     5142      -49     
==========================================
- Hits        33574    33573       -1     
- Misses      16397    16398       +1     
  Partials      131      131              

Flag	Coverage Δ
cypress	50.91% <ø> (-0.01%)	⬇️
javascript	61.73% <ø> (ø)
python	64.08% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...set-frontend/src/dashboard/util/getDropPosition.js	92.06% <0.00%> (-1.59%)	⬇️

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