DOC-3021: Update plugin documentation for open source and premium plu…

[jeslynbo]

Ticket: DOC-3021

Site: Plugins page (staging)

Changes:

• Reorganized premium and open source plugins into capability categories aligned with the tiny.cloud features page taxonomy (Core Editing, Collaboration, Content Conversion & Embedding, Compliance, Productivity, Document Organization), so a feature sits in the same category across the website and the docs. The same category vocabulary is applied to both the premium and open source sections.
• Recategorized the left-hand navigation (modules/ROOT/nav.adoc) into the same categories so the nav and the plugins page are consistent. All existing plugin and sub-page links are preserved.
• Added a dedicated AI category that surfaces TinyMCE AI, AI Chat, AI Quick Actions, and AI Review as separate entries rather than a single line. Removed the legacy AI Assistant from the index cards so it no longer overshadows TinyMCE AI; its documentation page and nav entry are retained.
• Removed the Premium card labels — the premium/open source split into separate sections already conveys the distinction, so the per-card badge was redundant.
• Fixed the two-column card tables dropping the last plugin when a category had an odd number of plugins. This restored Spell Checker, Suggested Edits, Tiny Drive, Templates, and Search and Replace to the rendered page.
• Fixed the right-hand table of contents so open source categories nest correctly under their section heading.
• Rewrote all plugin descriptions in a consistent indicative voice and removed promotional phrasing.
• Corrected description inaccuracies verified against tinymce and tinymce-premium source, including Enhanced Tables, Import from Word, Accessibility Checker, Visual Characters, Revision History, Full Page HTML, Media, Lists, Quick Toolbars, Autolink, and Word Count.
• Applied sentence case to headings and removed second-person phrasing on the plugins page.

Follow-up (separate repo): the roomier spacing between category headings requested in review needs a small CSS change to the category heading level in docs-antora-ui.

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• modules/ROOT/nav.adoc has been updated (if applicable).
• Included a release note entry for any New product features. (N/A — documentation restructure, no new product feature)
• If this is a minor release, updated productminorversion in antora.yml and added new supported versions entry in modules/ROOT/partials/misc/supported-versions.adoc. (N/A)
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed

[kemister85]

@jeslynbo

• We cannot remove ai legacy plugin as this is still available and used by customers.
• I do however like the approach of breaking these into their own groups like you have.
• One thing that is obvious to me is, "whats core vs premium"? someone visiting the site cannot tell whats a paid vs free open source feature?
  Just some early feedback 😄