Refresh pipeline migration and backend docs

[tomdurrant]

Summary

• replace the stale standalone pipeline migration guide with published CLI documentation and core rompy example configs
• add inline and include-based pipeline examples that use built-in base, local, and noop types instead of rompy-ww3 concepts
• expand the backends documentation with a fuller pipeline integration section and correct the programmatic API example

Testing

• not run; docs and example configuration updates only