fix website production css

[cheng-tan]

Why are these changes needed?

• Docusaurus production doesn't support nested css(Can't use CSS nesting in prod facebook/docusaurus#9303) adding in postcss plugin to compile the modern css to support older version browsers.
• Cleaned up the custom.css file a bit more and fixed two additional accessibility issue

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (05923a7) 32.45% compared to head (1baef35) 32.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1366   +/-   ##
=======================================
  Coverage   32.45%   32.45%           
=======================================
  Files          41       41           
  Lines        4905     4905           
  Branches     1119     1119           
=======================================
  Hits         1592     1592           
  Misses       3187     3187           
  Partials      126      126           

Flag	Coverage Δ
unittests	32.41% <ø> (ø)

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[victordibia]

Thanks @cheng-tan.
Looking good. Will double check that things compile successfully and then finalize my review

[victordibia]

I confirm I have tested this.
I really appreciate the updates to contrast that this fixes.

@sonichi , @qingyun-wu , I believe this is ready for merging.