docs: improve Python API reference UX

[viiccwen]

Related Issues

Closes #1357
Related to #1361

Changes

• Bug fix
• New feature
• Refactoring
• Documentation
• Test
• CI/CD pipeline
• Other

Why

The generated Python API references were published separately from the existing Qumat and QDP API documentation.

This resulted in duplicate API entries and made it unclear which reference users should open. The generated pages also used product-specific Python API titles even though their surrounding Qumat and QDP sections already provide that context.

The current documentation additionally contained stale /docs/next/ links and an outdated Qumat getting-started example.

How

• Generate the current Qumat API reference directly at qumat/api.
• Generate the current QDP API reference directly at qdp/api.
• Replace the hand-written API pages while preserving their established routes and sidebar locations.
• Remove the separate current-version API landing page and Python API sidebar category.
• Use API Reference consistently as the page title, heading, and sidebar label.
• Add clearer introductions, guide links, table-of-contents settings, and key QDP entry-point links.
• Disable edit links on generated API pages.
• Keep API placeholder pages available when generation is skipped for local previews.
• Update stale website paths and the Qumat getting-started example.
• Leave the released 0.6 documentation unchanged.

This PR does not include QDP source-docstring formatting changes; those are handled separately under #1358.

Validation

• npm run typecheck
• npm run build
• Confirmed the generated routes:
  • /docs/master/qumat/api
  • /docs/master/qdp/api
• Confirmed the obsolete /docs/master/api landing route is no longer generated.
• Confirmed both generated pages use API Reference for their title, heading, and sidebar label.
• Confirmed the QDP header and generated content are separated correctly in the resulting Markdown.
• Confirmed there are no changes to version 0.6 documentation, tests, CI workflows, package manifests, or qdp/qdp-python/qumat_qdp.

The website build succeeds with existing broken-link warnings from the unchanged 0.4 and 0.6 documentation snapshots.

Checklist

• Added or updated unit tests for all changes — not applicable; documentation and website integration only
• Added or updated documentation for all changes