Add a notebook demonstrating how to find and download files from ESGF

[bouweandela]

Description

Add a notebook that demonstrates the use of the esmvalcore.esgf module.

Notebook: https://github.com/ESMValGroup/ESMValCore/blob/add-esgf-files-notebook/notebooks/search-and-download-files.ipynb

Link to documentation: https://esmvaltool--2265.org.readthedocs.build/projects/ESMValCore/en/2265/notebooks/search-and-download-files.html

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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
• 🛠 Code is written according to the code quality guidelines
• 🧪 and 🛠 Documentation is available
• 🛠 Changes are backward compatible
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6e61590) 93.48% compared to head (f315b7c) 93.48%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2265   +/-   ##
=======================================
  Coverage   93.48%   93.48%           
=======================================
  Files         238      238           
  Lines       12932    12932           
=======================================
  Hits        12089    12089           
  Misses        843      843           

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