Totally new documentation

[mvertens]

Complete rewrite of the CMEPS documentation

With the help of Claude - this PR is a full rewrite and restructuring of the CMEPS Sphinx documentation
into four clearly-scoped parts.
See https://noresmhub.github.io/CMEPS/ to view the new documentation in this PR.

New structure

• Overview & Concepts — what the mediator is, the driver/mediator split, the run sequence & run configuration, import/export states, the three operations (map/merge/average), computed fields (atm/ocn fluxes, ocean
  albedos, land→land-ice downscaling), surface fractions, and conservation.
• User Guide — the run sequence, the run configuration, how a case is configured and run, and coupled field names.
• Developer Guide — code organization & internal state, field exchange (addfld/addmap/addmrg), conservation math, the phase lifecycle, mapping, merging, fractions, atm/ocn fluxes, ocean albedo, budgets, history,
  restart, and a walkthrough for adding a coupled field.
• Reference — field-name tables, required attributes (generic + CESM/NorESM), and a glossary.

Legacy content — migrated, not lost

The old pages were retired after their content was moved into the new structure:

Legacy page	Migrated to
introduction.rst	overview/concepts + developer/conservation
esmflds.rst	developer/field_exchange + users/fields
fractions.rst	developer/fractions
generic.rst	developer/code_organization
prep.rst	developer/phase_lifecycle
addendum/*	reference/*

Building the docs

cd doc
pip install "sphinx<8" sphinx-rtd-theme
make html
open build/html/index.html   # or xdg-open on Linux
The build is clean — a new CI workflow (.github/workflows/docs.yml) builds
the docs on every PR with warnings-as-errors and uploads the rendered HTML as a
downloadable artifact (cmeps-docs-html), so reviewers can view the pages
without building locally.

Notes for reviewers

• Please review the rendered HTML (via the artifact), not just the .rst diff — the layout and cross-links matter.
• The User Guide is intentionally scoped to CESM/NorESM; - the Overview and Developer guides are host-agnostic.

[mvertens]

@billsacks @uturuncoglu @DeniseWorthen - using help from Claude - I have totally rewritten the CMEPS documentation. I thought I would start with a PR to NorESMhub first - and then do a clean PR to ESCOMP. Please let me know what you want changed or rephrased. Your feedback would be super appreciated.
Please see the currently rendered new documentation here: https://noresmhub.github.io/CMEPS/

[uturuncoglu]

@mvertens Thanks for doing this. We were desperately need this but could not find time for it. Let me go through the documentation. I'll update you about it soon.

[mvertens]

@uturuncoglu - thank you! Note that the User Guide for now is CESM/NorESM specific (that's stated at the top) - but it would be great to add a UFS specific section(s). Please feel free to suggest any changes - and do a PR to my branch on NorESMhub for now.

[mvertens]

@uturuncoglu - I have also added some place holders for UFS in the User's Guide - so it's not longer just CESM/NorESM specific.

[uturuncoglu]

@mvertens Perfect. Thanks. I am still reviewing it. I'll push my review until end of this week.

[mvertens]

@anton-seaice - adding you to this as well - since this documentation might be valuable for your project and it would be good to possibly your model specific uses?