Documentation layout to highlight 2 packages

[flicj191]

Description

For parallel updates to ESMValTool documentation:
ESMValGroup/ESMValTool#3914

Closes issue: ESMValGroup/ESMValTool#3860

Link to documentation:

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Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

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• 🛠 Unit tests have been added
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.41%. Comparing base (181f18d) to head (30b33a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2732   +/-   ##
=======================================
  Coverage   95.41%   95.41%           
=======================================
  Files         260      260           
  Lines       15409    15409           
=======================================
  Hits        14703    14703           
  Misses        706      706           

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[flicj191]

Currently just has ESMValCore for the .. toctree:: could still include the generation to list ESMValTool headings as well.
Quick schema diagram needs reviewing for consistency