Synchronize initial documentation page

[bouweandela]

Description

Update the main documentation page, which was updated in ESMValGroup/ESMValTool#3628, so it looks the same when viewing the ESMValCore subproject.

Link to documentation: https://esmvaltool--2464.org.readthedocs.build/projects/ESMValCore/en/2464/

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Checklist

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• 🧪 The new functionality is relevant and scientifically sound
• 🛠 This pull request has a descriptive title and labels
• 🧪 and 🛠 Documentation is available
• 🛠 Changes are backward compatible
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.62%. Comparing base (d6eaac2) to head (7b31405).
Report is 34 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2464   +/-   ##
=======================================
  Coverage   94.62%   94.62%           
=======================================
  Files         246      246           
  Lines       14055    14055           
=======================================
  Hits        13299    13299           
  Misses        756      756           

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[valeriupredoi]

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