doc: add note on identity observations

[hkershaw-brown]

Description:

Added a paragraph on Identity observations in the first section in the docs under 'Observations'

Not sure if there is a better place to put it, e.g. in "introduction to ensemble data assimilation"
https://docs.dart.ucar.edu/en/latest/guide/introduction-ensemble-da.html

Used big H for forward operator, not sure if this is convention in DA.

Fixes issue

fixes #1037 Identity obs not mentioned in the docs at all, recently had support requests asking about identity obs and types.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes.
Built the docs locally.

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed

[hkershaw-brown]

@kdraeder is this something you have time to review? Cheers, Helen