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 functional categories.
• Added a styled Premium label to premium plugin cards and the TinyMCE AI banner, replacing the plain (Premium) text.
• 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.
• Returned the AI Assistant (legacy) plugin and cross-referenced it to TinyMCE AI.
• 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.

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.
• 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.
• 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 😄