[yaml] Enhance YAML API docs

[Polber]

The current YAML API refernce docs look like the following:

This PR adds formatting to make the docs look more like the Sphinx-themed PyDocs used by the Python SDK:

This change not only adds some color and easier-to-read formatting , but also separates the transforms into individual pages to reduce clutter. This should hopefully make the user-experience a bit better.

UPDATE: the doc will remain as one page, but will include the new side panel and formatting changes.

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[Polber]

R: @robertwb

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 60.44%. Comparing base (67dc973) to head (a8987bd).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/yaml/generate_yaml_docs.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #32825      +/-   ##
============================================
+ Coverage     60.39%   60.44%   +0.04%     
  Complexity    15099    15099              
============================================
  Files          2735     2736       +1     
  Lines        265201   265331     +130     
  Branches      12106    12106              
============================================
+ Hits         160171   160378     +207     
+ Misses        98610    98533      -77     
  Partials       6420     6420              

Flag	Coverage Δ
python	81.59% <0.00%> (+0.14%)	⬆️

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