docs: create a dedicated page in our docs for the Prometheus integration

[asaphko]

Currently, the Prometheus integration is mentioned mid-page on the Grafana integration page - https://docs.flagsmith.com/third-party-integrations/observability-and-monitoring/grafana.

The title of this page in the sidebar is "Grafana & Prometheus" but the page title itself is only "Grafana" and Prometheus is only mentioned inline.

UAC:

1. the Prometheus integration has a dedicated page in our docs, with a link from the sidebar (under Grafana),
2. the sidebar link that is now "Grafana & Prometheus" changes to only say "Grafana"
3. The main "Third-Party Integrations" page is updated to show links to both pages in the "Observability & Monitoring" section (correcting the current "Grafana & Prometheus Integration" to only Grafana)

[khvn26]

🤔 The Third-Party Integrations section documents integration that operate user data — i.e. the Grafana integration provides audit log-based annotations, and Sentry integration exports segment/feature changes and/or evaluation data.

The Prometheus integration does not operate on user data, however; rather, it provides observability for those operating the Flagsmith deployment. This is why it's documented under Deployment and Self-hosting / Observability / Metrics section.

[asaphko]

It is already listed there - i'm just asking to split it into its own page.

[khvn26]

Ah, I think the actual Prometheus integration in question is the Feature Health one. I understand now, thanks 👍