[DOC] PyPI-first install docs; drop unused dolfyn dependency

[eleanorfrajka]

Summary

Reworks the install documentation now that both oceanarray and seasenselib are on PyPI, and removes the redundant direct dolfyn dependency.

seasenselib is now a hard dependency (declared in requirements.txt, which feeds pyproject.toml's dynamic dependencies), so the old pip install --no-deps / pip install -e ../seasenselib workaround is gone. A plain pip install oceanarray resolves the whole stack, verified end-to-end in a fresh venv (imports plus oceanarray --help and seasenselib list).

Changes

requirements.txt: seasenselib becomes a hard dependency; the direct dolfyn line is removed. oceanarray never imports dolfyn — the rdi-raw reader lives in seasenselib, which reads RDI files through mhkit.dolfyn (mhkit vendors dolfyn as a subpackage and is pulled in via seasenselib's mhkit[dolfyn]>=1.0.0). The standalone dolfyn package was installed but imported by nothing. Verified: with standalone dolfyn uninstalled, seasenselib's reader still resolves via mhkit.dolfyn.io.rdi.read_rdi and oceanarray.processors.stage1 imports cleanly.

docs/source/setup.md, quickstart.rst, yaml_configuration.rst, roadmap.rst, README.md, troubleshooting.rst: install instructions lead with pip install oceanarray; the separate "install mhkit[dolfyn] for RDI" steps are dropped because RDI support now arrives automatically with seasenselib; the roadmap dependency list is corrected (dolfyn removed as a direct dep; rdi-raw added to the seasenselib reader list).

Notes on dependency footprint

Making seasenselib a hard dependency raises the download from ~81 MB (core) to ~236 MB. The heaviest transitive additions — PyQt5/Qt (~42 MB, via pylablib) and llvmlite/numba (~40 MB, via pylablib) — are hopefully being removed upstream in seasenselib; mhkit→pyarrow (~34 MB) stays because it carries the vendored dolfyn used for RDI reading. After the upstream trim the seasenselib delta drops to ~73 MB, which is why seasenselib is kept as a hard dependency rather than an optional [raw] extra.

Breaking changes

None. seasenselib moving from a manual side-install to a declared dependency only adds packages to a fresh install; no public API, CLI flag, output layout, or config option changes. Removing the direct dolfyn line changes nothing observable — RDI reading already went through seasenselib's mhkit.dolfyn.