doc: default model_mod documentation

[hkershaw-brown]

Description:

This pull request updates the default_model_mod documentation. Previously the docs said the routines do no work, but some do! The default model mod routines and their outputs are described in "required model_mod routines" so this file has been updated. Removing the out-of-date template/readme.rst required updating filter.rst and closest_member_tools.rst to remove the now broken link.

Changes:

Updated “New Models” section in filter.rst to direct people to the “Porting new models guide.” This change removed the broken links in the paragraph.

Template model:

• Added new_model.rst to the ignore list, this file is a template used in the creation of a new model directory, not part of the documentation.
• Removed template/readme.rst as this was out of date and the information is covered in "required model_mod routines"

Updated “Required model_mod routines”

• pert_model_copies as the example as this is more likely to be relevant than init_time
• Removed the directory location from the table, not important for user who will “use default_model_mod” to get the routines
• Changed the table format to a list table so entries can be edited more easily

Default model mod documentation:

• Changed title to default_model_mod rather than just model_mod.
• Concrete example of using a routine.
• Note on the “fail” routines

Fixes issue

Fixes #798

Partial fix for #957 (new model section)

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 documentation locally.
make linkcheck to find broken links

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

[mgharamti]

I didn't see any issues here. This seems to be a great documentation update for model_mod-related stuff. I do appreciate the direct link to porting instructions from filter page.