Propose placement for OTLP → Prometheus push flow documentation

[Wineshuga]

This is a follow-up to #9520

Proposal to add a new page at docs/compatibility/prometheus.

During review of PR #9570, it was suggested that the OTLP-Prometheus push flow be moved to a more appropriate location in the documentation.

This content fits under docs/compatibility/prometheus because it is an integration between OpenTelemetry and Prometheus. It describes interoperability between the two systems and is better grouped with other cross-system guidance rather than _includes.

Users looking for an OTLP-Prometheus setup naturally expect this kind of content in the compatibility or integration sections.

Please let me know if this should live elsewhere or if additional adjustments are needed.

[tiffany76]

Hi @Wineshuga! Thanks for raising this issue. Your proposal makes sense to me, but we should hear from the @open-telemetry/prometheus-interoperability folks.

[ArthurSens]

Agreed, this would be a good addition to the docs, and the new compatibility section looks like a good place :)

Just need to figure out how to organize the subsections in a way that makes sense