docs: add InvokeAI Integration user guide page

[lstein]

Summary

• Adds docs/user-guide/invokeai-integration.md, a new User Guide page covering:
  • what InvokeAI is and where to get it
  • creating a PhotoMapAI album from <invokeai-root>/outputs/images/ (with a note about manual re-indexing)
  • configuring the InvokeAI Backend URL in Settings, including optional multi-user credentials and the upload board selector
  • the Recall, Remix, and Use as Ref Image actions in the metadata drawer
  • troubleshooting common URL / auth / cross-machine issues
• Adds five screenshots under docs/img/.
• Slots the new page into the User Guide nav in mkdocs.yml.
• Enables the admonition, pymdownx.details, and pymdownx.superfences markdown extensions so future pages can use boxed notes / collapsible blocks.
• Drops the explicit --dev-addr=0.0.0.0:8000 from make docs (defaults are fine for the typical workflow).
• Pins mkdocs<1.6 in the development extras to work around the upstream live-reload regression introduced when mkdocs 1.6 swapped its dev-server backend.
• Minor copy-edit on docs/index.md (mentions InvokeAI integration in the feature list) and a stale link fix in docs/user-guide/bookmarks.md.

Test plan

• python -m mkdocs build --clean produces no warnings.
• make docs serves the site locally and the new page renders correctly with all screenshots and intra-doc links.
• Browsing to User Guide → InvokeAI Integration in the rendered nav loads the page.
• Each anchor / cross-link in the page resolves (especially the link to Managing Albums → Indexing Albums and the in-page Troubleshooting anchor).

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