Rename service docs file (postgres-exporter.md -> prometheus-postgres-exporter.md) and announce on services list

Related to https://github.com/mother-of-all-self-hosting/mash-playbook/pull/58

Renaming because:

- we already have other Prometheus exporters, which follow the `prometheus-*-exporter` naming

- being more explicit is clearer in any case

- the role is named `*-prometheus-postgres-exporter`, so it's also
  consistent with that

docs/supported-services.md

@@ -33,8 +33,9 @@
 | [Postgres](https://www.postgresql.org) | A powerful, open source object-relational database system | [Link](services/postgres.md) |
 | [Postgres Backup](https://github.com/prodrigestivill/docker-postgres-backup-local) | A solution for backing up PostgresSQL to local filesystem with periodic backups. | [Link](services/postgres-backup.md) |
 | [Prometheus](https://prometheus.io/) | A metrics collection and alerting monitoring solution | [Link](services/prometheus.md) |
-| [Prometheus Node Exporter](https://github.com/prometheus/node_exporter) | Exporter for machine metrics | [Link](services/prometheus-node-exporter.md) |
 | [Prometheus Blackbox Exporter](https://github.com/prometheus/blackbox_exporter) | Blackbox probing of HTTP/HTTPS/DNS/TCP/ICMP and gRPC endpoints | [Link](services/prometheus-blackbox-exporter.md) |
+| [Prometheus Node Exporter](https://github.com/prometheus/node_exporter) | Exporter for machine metrics | [Link](services/prometheus-node-exporter.md) |
+| [Prometheus Postgres Exporter](https://github.com/prometheus-community/postgres_exporter) | A PostgreSQL metric exporter for Prometheus | [Link](services/prometheus-postgres-exporter.md) |
 | [Radicale](https://radicale.org/) | A Free and Open-Source CalDAV and CardDAV Server (solution for hosting contacts and calendars) | [Link](services/radicale.md) |
 | [Redmine](https://redmine.org/) | A flexible project management web application. | [Link](services/redmine.md) |
 | [Redis](https://redis.io/) | An in-memory data store used by millions of developers as a database, cache, streaming engine, and message broker. | [Link](services/redis.md) |