Fix AsciiDoc build warnings and broken cross-references

[kemister85]

Ticket: N/A — documentation build hygiene (no associated ticket)

Site: Staging branch

Changes:

Resolves all Antora/Asciidoctor build warnings on the tinymce/8 line (26 → 0) and repairs a set of broken cross-references. Verified with a full build: 0 errors, 0 warnings.

• Escaped literal curly-brace tokens that were being parsed as attribute references (and dropped from output): REST/MCP path parameters {version}, {serverName}, {AI_SERVICE_URL}; the trial placeholder {apiKey}; and the Handlebars example {{FooBar}}.
• Corrected heading levels in the TinyMCE AI commands partial (sections were one level too deep under "TinyMCE AI").
• Repaired broken cross-references — mismatched anchor ids, a missing .adoc extension, and wrong target pages in shared partials.

Visual review — staging deep links

Each link opens the changed section on the staging preview (available once the preview build finishes). Escaped tokens should now render literally (for example GET /v1/models/{version}); repaired links should resolve to a real target.

Curly-brace escapes

• tinymceai-models.adoc → Verifying limits — {version} in the GET /v1/models/{version} paths
• tinymceai-limits.adoc → File limits — {version}
• tinymceai-chat.adoc → Model selection — {version}
• tinymceai-api-quick-start.adoc → Next steps — {version}
• tinymceai-jwt-authentication-intro.adoc → Quick start — {apiKey}
• tinymceai-on-premises-mcp.adoc → Proxy endpoints — {serverName} / {AI_SERVICE_URL} in the four proxy routes
• 8.7.0-release-notes.adoc → CSS custom property section — {{FooBar}} example

Heading levels

• tinymceai-cmds.adoc → Toggling sidebars and Plugin commands — should nest correctly under TinyMCE AI

Repaired cross-references

• editor-command-identifiers.adoc → Plugin Commands list — Export to PDF / Export to Word / Import from Word now jump to their sections; Enhanced Tables anchor renamed
• license-key.adoc → Load error (Cloud) and Load error (Self-Hosted) — table links now resolve
• full-featured-open-source-demo.adoc — "Full featured demo including Premium Plugins" link in the excluded-plugins table resolves
• smart_paste.adoc → in Copy and paste — powerpaste_autolink_urls note links to powerpaste-options
• newdocument_content.adoc → in Content behavior — editable_root link points to non-editable-content-options

Pre-checks:

• Branch is correctly prefixed: hotfix/8/docs-build-warnings
• Build passes without console errors, warnings, or issues.
• modules/ROOT/nav.adoc has been updated (not applicable).
• Files added for new product features include a release note entry (not applicable).