Update documentation on derived variables with new CMOR tables configuration

[schlunma]

Link to docs: https://esmvaltool--2979.org.readthedocs.build/projects/ESMValCore/en/2979/develop/derivation.html

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.

• 🧪 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
• 🛠 Unit tests have been added
• 🛠 Changes are backward compatible
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.68%. Comparing base (da81d5f) to head (a3f42e7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2979   +/-   ##
=======================================
  Coverage   95.68%   95.68%           
=======================================
  Files         267      267           
  Lines       15730    15730           
=======================================
  Hits        15052    15052           
  Misses        678      678           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[bouweandela]

thanks!