TINYDOC-3629: TinyMCE 8.9.3 security release notes.

[kemister85]

Ticket: TINYDOC-3629

Site: Staging branch

Changes:

• Add 8.9.3-release-notes.adoc with the media plugin data-mce-object serialization XSS fix (GHSA-mf2p-h6hf-fcwm); CVE pending.

Pre-checks:

• Branch is correctly prefixed:
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files have been included where required (if applicable).
• Files added for New product features include a release note entry.
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.