Add documentation for undocumented interfaces

Not every important interface that's being actively used in the charming ecosystem has been formally documented. As we move towards migrating interface libraries to this repository (especially 'public' interfaces), and documenting all interfaces (again, especially 'public' ones), it would be great to add docs for these interfaces.

### Observability

[This comment](https://github.com/canonical/charmlibs/pull/274#discussion_r2582708537) lists the following undocumented interfaces:
> - [ ] alertmanager-k8s-operator
> - [ ] alertmanager_remote_configuration
> - [ ] alertmanager_dispatch
> - [ ] blackbox-exporter-k8s-operator
> - [ ] blackbox_probes
> - [ ] catalogue-k8s-operator
> - [ ] catalogue
> - [ ] cos-coordinated-workers
> - [ ] cluster
> - [ ] grafana-cloud-integrator
> - [ ] grafana_cloud_config
> - [ ] karma-k8s-operator
> - [ ] karma_dashboard
> - [ ] loki-k8s-operator
> - [ ] loki_push_api

And [this comment](https://github.com/canonical/charmlibs/pull/274#discussion_r2582676211) also mentions:
> Perhaps we need `prometheus_api` under `mimir_coordinator_k8s.v0` here as well? [canonical/mimir-coordinator-k8s-operator#115](https://github.com/canonical/mimir-coordinator-k8s-operator/pull/115)
> There is a small description for this lib here: https://charmhub.io/mimir-coordinator-k8s/libraries/prometheus_api

Start with the two linked discussions on pull request 274 and compare the listed interfaces with the existing charmlibs documentation. Review the Charmhub description for prometheus_api and the linked mimir-coordinator-k8s-operator pull request for context. Done means the undocumented interfaces identified here have suitable documentation, including any confirmed prometheus_api entry.

Written by the indexing model from the issue text.