[Carwyn Pelley (cpelley)]

Summary

Add dagrunner-plot to generate linked HTML workflow visualisations with a collapsed overview, per-attribute detail pages, and a configuration-selection index.

• Configure titles, graph collapsing, labels, grouping, and detail selection through JSON.
• Install the plotting command and browser assets without introducing a separate visual Python package.
• Support parent-controlled iframe themes while retaining the theme button for standalone graphs.
• Improve navigation-link spacing and document component usage and integration APIs.
• Expose get_networkx for reuse and handle empty label/group settings.
• Fix Google-style docstring parsing for parameters without explicit type annotations, with regression coverage.

Validation

• Docstring and CLI parser tests
• Built and installed a wheel; verified package contents and dagrunner-plot --help.
• Browser-tested theme synchronization, standalone controls, navigation, persistence, and responsive layout.
• Utilised by https://github.com/MetOffice/improver_suite/pull/2873