TINYDOC-3617: Backfill missing page description and navigation title attributes.

[kemister85]

Ticket: TINYDOC-3617

Site: Staging branch

Changes:

• modules/ROOT/pages/*.adoc: added :navtitle: to 29 pages and :description: to index.adoc and multiple-editors.adoc; corrected the misspelled :descripton: attribute on spell-checking.adoc.
• modules/ROOT/nav.adoc: changed four navigation labels to sentence case so they match the new navigation titles.
• modules/ROOT/pages/tinymceai-*.adoc: moved the attribute block directly under the title on eight TinyMCE AI pages, where a blank line had kept their description and navigation title out of the document header; sentence-cased five of those navigation titles and the four matching nav.adoc labels.

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How to test locally:

git fetch origin && git switch hotfix/8/TINYDOC-3617
npx antora ./antora-playbook-local.yml   # builds this branch only, about 10 s
npx http-server build/site -p 4000

Open http://localhost:4000/tinymce/latest/ and check:

• The navigation shows the sentence-case labels: Cloud troubleshooting, Cloud deployment of plugins only, Configure Enhanced Media Embed server, Integrate Enhanced Media Embed server, and under TinyMCE AI AI models, Integration options, API overview, API quick start.
• The page source of spell-checking/, multiple-editors/, tinymceai-models/ and tinymceai-api-overview/ contains <meta name="description" …>, and the start page (/tinymce/latest/) does too.
• git diff origin/tinymce/8 --stat lists only modules/ROOT/pages/*.adoc and modules/ROOT/nav.adoc; no partials.

The browser tab title and page heading do not change: they come from each page's = Title line, which this PR does not touch.

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Pre-checks:

• Branch is correctly prefixed:
• modules/ROOT/nav.adoc has been updated (if applicable).
• Build passes without console errors, warnings, or issues.

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Review:

• Documentation Team Lead has reviewed.