TINYDOC-3528: Added configurable welcome actions to the TinyMCE AI Chat empty state

[kemister85]

Ticket: TINYDOC-3528 (TINYMCE-14508)

Site: Staging

Changes:

• Added an Additions release note entry for TINYMCE-14508 to modules/ROOT/pages/8.8.0-release-notes.adoc: Added configurable welcome actions to the TinyMCE AI Chat empty state.
• Documented the new tinymceai_chat_welcome_actions configuration option in modules/ROOT/partials/configuration/tinymceai_options.adoc (type, default, item shapes, and a working example), with the release note xref-ing to it.
• Updated the TinyMCE AI chat live demo (modules/ROOT/examples/live-demos/tinymceai/index.js and example.js) to configure tinymceai_chat_welcome_message and tinymceai_chat_welcome_actions.

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Pre-checks:

• Branch is correctly prefixed (release-note branch)
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files have been included where required (the options partial is already included via tinymceai.adoc).
• Files removed have been deleted, not just excluded from the build (if applicable).
• Files added for New product features include a release note entry.
• Major or minor version changes have updated the supported-versions.adoc table.
• Build passes without console errors, warnings, or issues.