chore: port the full Pathway Browser userguide content from reactome.org

The /documentation/userguide/pathway-browser page had a stub of the
prod userguide. Pulled the article body from
https://reactome.org/userguide/pathway-browser verbatim and converted
HTML -> MDX:

  - Images rewritten from /images/userguide/pathway_browser/ to the
    /uploads/documentation/userguide/pathway-browser/ tree (which
    already has all 15 referenced assets, both the named PNGs and
    the hashed Google-doc imports that had been baked in earlier).
  - Legacy URL prefixes (/userguide, /dev, /cite, /icon-info,
    /linking-to-us) rewritten to /documentation/* to match the
    sweep in ed605c8.
  - All Joomla-template noise (template-scoped spans, classes,
    inline styles) stripped.

Verified: 15 .png/.gif images render via headless; section headings
("Hierarchy Panel", "Subpathway", "Reaction Node", etc.) all
present.

Author: adamjohnwright