Synchronize initial documentation page

[bouweandela]

Description

Update the main documentation page, which was updated in https://github.com/ESMValGroup/ESMValTool/pull/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

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.

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

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.62%. Comparing base (d6eaac2) to head (7b31405).

Additional details and impacted files

```diff @@ 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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