[MansurJisan-NCO]

Description

Folds the UFS-Coastal content into the Systems section instead of keeping it separate. The text is unchanged; only the headings and its location change.

• README.md: the "UFS-Coastal files" section becomes a "UFS-Coastal" subsection under "Systems".
• docs: docs/ufs_coastal.rst is removed. Its content becomes a "UFS-Coastal" subsection under "Supported systems" in docs/index.rst. The toctree entry and the duplicate Key-features bullet are removed.

Type of change

• Maintenance (refactor, cleanup, CI, comments)

Change characteristics

• Changes outputs (values, COMOUT files or names, archives)? NO
• Breaking change to existing functionality? NO
• Requires a nos-utils submodule bump (nos-workflow) or a paired nos-workflow change (nos-utils)? NO
• Requires new or regenerated fix files on the machine? NO
• Requires a re-prep or a new hotstart before the next cycle? NO
• Machines affected: none (docs only)

How has this been tested?

• The moved text was checked to be byte-identical to the reviewed text from docs: list the UFS-Coastal files nos-utils generates #77, in both the README and the RST.
• No references to the removed page remain.
• Sphinx build with a Read the Docs-style install: no new warnings, and the ufs-coastal anchor is present on the index page.

Checklist

• Dependent PRs merged and published (nos-utils before any submodule bump)
• pytest suite passes
• Existing operational behavior preserved, or the change is called out above
• No edits to a shared FIXofs while it is cycling
• Self-review done; yaml comments and docstrings updated where behavior changed
• No markdown files added to the repo (this template is the only exception)