Fix documentation access and navigation, and add indexed search

[bonelifer]

Summary

• Fix ACP menu depth, route-base handling, permission checks on fallback pages and images, and dynamic permission cleanup on purge.
• Add full-text documentation search, optional navigation icons, external-menu support and a master documentation switch.
• Improve sidebar filtering, page navigation, tables, styles and accessible tooltips. Preserve image descriptions without changing PO catalogs.
• Restore PHPUnit discovery and add focused controller, image, listener, purge and ACP template tests. Archive the historical review while keeping unresolved upstream TODOs.

Test plan

• Codex reran the local PHPUnit suite: 81 tests, 211 assertions passed against phpBB dependencies.
• PHP syntax checks and git diff --check passed. The six-language Hugo build passed.
• Earlier local browser checks covered public documentation, tables and tooltip interactions; live ACP tooltip behavior was not verified.
• The missing upstream development images remain tracked TODOs, not claimed fixes.

cc @kaileymsnay

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Investigated and written by Codex on behalf of William Jacoby (bonelifer).