Extend Documentation about schema files

[Ozaq]

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📚 Documentation preview 📚: https://ecmwf-fdb--125.com.readthedocs.build/en/125/

[codecov-commenter]

Codecov Report

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

Project coverage is 63.55%. Comparing base (e43231b) to head (23ace31).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #125   +/-   ##
========================================
  Coverage    63.55%   63.55%           
========================================
  Files          281      281           
  Lines        15914    15914           
  Branches      1643     1643           
========================================
  Hits         10114    10114           
  Misses        5800     5800           

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