docs: publish an OBS and FFmpeg interoperability matrix

[AlexanderWagnerDev]

Goal

Create a public compatibility matrix that summarizes tested publisher → librtmp2/server → player combinations without overstating parser-only support.

Why

Users currently have to infer compatibility from implementation-status tables, tests, and individual issues. A concise matrix would make the supported RTMP/RTMPS and codec paths easier to evaluate and would turn the new interoperability issue form into reusable evidence.

Proposed scope

Add docs/interoperability.md with columns for:

• OpenRTMP version/commit
• Publisher and version
• Player/downstream consumer and version
• RTMP or RTMPS
• Video and audio codec
• Initial publish/play result
• Late player join
• Publisher reconnect
• Multiple players
• Notes/workarounds
• Evidence issue or test link

Include a clear legend for verified, partial, failed, and not tested.

Acceptance criteria

• Matrix distinguishes legacy RTMP, enhanced media passthrough, and full session-level features.
• Every verified row links to a reproducible test, CI case, or sanitized interoperability report.
• No row treats parser-only code as end-to-end support.
• README links to the matrix near the implementation-status section.
• Contribution instructions explain how to add a row.

Starter task

Begin with combinations already covered by repository tests and clearly mark all unverified scenarios as not tested.