TINYDOC-3639: New multi-editor support for TinyMCE AI.

[kemister85]

Ticket: TINYDOC-3639 (TINYMCE-14817)

Site: Staging

Changes:

• New TinyMCE AI multi-editor mode page with two live demos (one group, and two independent groups).
• New tinymceai_group, tinymceai_document_name, and tinymceai_document_description options.
• Release note entry for TINYMCE-14817 in the 8.10.0 release notes.

Main pages changed:

• Multi-editor mode
  • New page: shared sidebar, chat, review, quick actions, previews, integration, and multiple groups.
• Options for multi-editor mode
  • The three new options, with a two-group example for tinymceai_group.
• 8.10.0 release notes
  • New multi-editor support entry under TinyMCE AI.

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Pre-checks:

• Branch is correctly prefixed: feature/8.10/TINYDOC-3574_TINYMCE-14817
• modules/ROOT/nav.adoc has been updated.
• Files have been included where required.
• Files added for New product features include a release note entry.
• Build passes without console errors, warnings, or issues.

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Review:

• Documentation Team Lead has reviewed.