TINYDOC-3562 - Document beforeSetContent listener for AI-only revision history

[kemister85]

Ticket: TINYDOC-3562

Site: Staging branch

Changes:

• Document that the TinyMCE AI plugin sets ai to true on both the BeforeSetContent and SetContent events, so either event can signal AI involvement.
• Explain both events up front in the "Attributing AI-assisted revisions" introduction, immediately after the note that the Revision History plugin does not set the marker automatically, rather than introducing BeforeSetContent later as an addition.
• Present the two events as equally valid choices: SetContent alone records the AI-assisted result, and listening to both also preserves the content that preceded each AI edit as its own revision. The choice is left to the integrator's use case.
• Add a BeforeSetContent example alongside the existing SetContent example in "Saving an AI-assisted revision", showing how to capture the pre-AI content via editor.getContent().

Pre-checks:

• Branch is correctly prefixed: hotfix/8/TINYDOC-3562
• Files have been included where required (if applicable).
• Build passes without console errors, warnings, or issues.

Prepared for documentation team review.

[metricjs]

A couple details need clarification, and the examples aren't consistent - left comments. But almost there

[metricjs]

Do we still need both these examples really? I expected either one example with both events, or one example for each event. This structure aligns more with the old structure of the section, with the initial focus on just setcontent