TINYDOC-3644: Add missing pages to the navigation and remove the empty Import from Word stub page.

[kemister85]

Ticket: TINYDOC-3644

Site: React technical reference (staging)

Changes:

• modules/ROOT/nav.adoc: added an Integration technical references group under Installation and the Custom Icon Pack Demo under Skins and Icons examples, and fixed the broken "What we do to maintain security" anchor.
• Removed the empty import-from-word-standalone-service.adoc stub and added a page-aliases redirect from it to docx-to-html-converter-api.adoc.

Pre-checks:

• Branch is correctly prefixed:
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files removed have been deleted, not just excluded from the build (if applicable).
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.