feat(docs): unify CSS for multiple mkdocs projects

[ecormany]

Overview

Adding a lil plugin so we can share one CSS file between multiple publications. You'd think this would work out of the box, but it totally doesn't!

Test Plan and Hands on Testing

tested locally, will see if this works in CI
…it does!

Changelog

• vibecoded a new plugin
• moved opentrons-theme.css up
• added the new plugin to Flex and Stacker docs yml
• pointed to the new css location
• added the plugin as editable load in uv setup

Review requests

• where does this fit on the spectrum of kludge to elegant?
• whomst should we list as plugin author?

Risk assessment

low. plugin code created from whole cloth by gemini but looks very straightforward.

[sfoster1]

couple improvements to make for readability, consistency, and reliability

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.75%. Comparing base (981511a) to head (cc711dc).
Report is 3 commits behind head on edge.

Additional details and impacted files

@@           Coverage Diff           @@
##             edge   #18670   +/-   ##
=======================================
  Coverage   24.75%   24.75%           
=======================================
  Files        3284     3284           
  Lines      285464   285416   -48     
  Branches    28663    28655    -8     
=======================================
  Hits        70662    70662           
+ Misses     214781   214733   -48     
  Partials       21       21           

Flag	Coverage Δ
protocol-designer	19.04% <ø> (+<0.01%)	⬆️
step-generation	5.25% <ø> (+<0.01%)	⬆️

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