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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (84cfe15) to head (8b69fad).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2732   +/-   ##
=======================================
  Coverage   95.46%   95.46%           
=======================================
  Files         260      260           
  Lines       15519    15519           
=======================================
  Hits        14815    14815           
  Misses        704      704           

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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

[bettina-gier]

@lukruh, @ehogan Can you check if this PR is equivalent to the tool one and has all your comments incorporated so we can merge this simultaneously (and soon)?

[lukruh]

The docs build fine. Landing page, header and sidebars are similar to the PR in the tool. The only difference I noticed were the icons in the header:

Both versions are fine (I personally prefer the simple icons as they are here), but would be nice to have them consistent.
@flicj191 if you are ok with me committing the other minor comments, I can do so and this can be merged.