docs: add dev note about feature toggles

[nsprenkle]

TL;DR - I wanted a note somewhere about where ORA-specific feature toggles live. I added a brief note.

JIRA: https://openedx.atlassian.net/browse/AU-284

Developer Checklist

• Reviewed the release process
• n/a Translations and JS/SASS compiled
• n/a Bumped version number in setup.py and package.json

FYI: @edx/masters-devs-gta

[pomegranited]

👍 better late than never?

• I tested this by reading the rendered docs change, and verified that the added documentation is still correct.
• I read through the code
• Updates documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.29%. Comparing base (a1710f0) to head (ab433e3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1694   +/-   ##
=======================================
  Coverage   95.29%   95.29%           
=======================================
  Files         195      195           
  Lines       21624    21624           
  Branches     1502     1502           
=======================================
  Hits        20606    20606           
  Misses        771      771           
  Partials      247      247           

Flag	Coverage Δ
unittests	95.29% <ø> (ø)

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