docs: ADR to expose all SCSS variables that MFEs use as CSS variables [BB-6307]

[xitij2000]

Description

Adds an ADR for a plan to expose all Bootstrap variables used by Paragon and MFEs as CSS variables.

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[codecov[bot]]

Codecov Report

Base: 90.97% // Head: 90.36% // Decreases project coverage by -0.61% :warning:

Coverage data is based on head (d711d2e) compared to base (45887c9). Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1388      +/-   ##
==========================================
- Coverage   90.97%   90.36%   -0.62%     
==========================================
  Files         195      209      +14     
  Lines        3170     3581     +411     
  Branches      715      838     +123     
==========================================
+ Hits         2884     3236     +352     
- Misses        273      330      +57     
- Partials       13       15       +2     

Impacted Files	Coverage Δ
src/Tabs/Tab.jsx	50.00% <0.00%> (-50.00%)	:arrow_down:
src/Nav/index.jsx	50.00% <0.00%> (-50.00%)	:arrow_down:
src/Navbar/index.jsx	52.17% <0.00%> (-32.04%)	:arrow_down:
src/Modal/ModalLayer.jsx	68.42% <0.00%> (-31.58%)	:arrow_down:
src/Button/index.jsx	73.33% <0.00%> (-26.67%)	:arrow_down:
src/Popover/index.jsx	75.00% <0.00%> (-25.00%)	:arrow_down:
src/Overlay/index.jsx	81.81% <0.00%> (-18.19%)	:arrow_down:
src/Modal/AlertModal.jsx	33.33% <0.00%> (-16.67%)	:arrow_down:
src/Modal/StandardModal.jsx	50.00% <0.00%> (-16.67%)	:arrow_down:
src/Modal/FullscreenModal.jsx	50.00% <0.00%> (-16.67%)	:arrow_down:
... and 100 more

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[natabene]

@xitij2000 Thank you for the contribution!

[xitij2000]

I've created a starting implementation draft PR for this: https://github.com/openedx/paragon/pull/1620

[openedx-webhooks]

@xitij2000 Even though your pull request wasn’t merged, please take a moment to answer a two question survey so we can improve your experience in the future.