docs: UI refactor ADR

[nsprenkle]

TL;DR - Add ADR for UI mixin refactors.

What changed?

• Add ADR 003
• Link repo docs in README.rst

Developer Checklist

• [x] Reviewed the release process
• [ ] Bumped version number in openassessment/__init__.py and package.json

Testing Instructions

n/a

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• [ ] I've done a visual documentation review

FYI: @openedx/content-aurora

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b2b9694) 95.04% compared to head (add7720) 95.04%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2049   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files         178      178           
  Lines       18145    18145           
  Branches     1656     1656           
=======================================
  Hits        17246    17246           
  Misses        677      677           
  Partials      222      222           

Flag	Coverage Δ
unittests	95.04% <ø> (ø)

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