Feedback on /admin/monitoring/prometheus-jmx-exporter.md: Improve usability

### Documentation feedback

- Page title: Prometheus JMX Exporter
- Page URL: https://cratedb.com/docs/guide/admin/monitoring/prometheus-jmx-exporter.html
- Source: https://github.com/crate/cratedb-guide/blob/490/docs/admin/monitoring/prometheus-jmx-exporter.md

---

This request might sound silly after so many years of existance with the status quo, but improving the enablement of the JMX exporter like this would tremendously enhance usability:

a) Ship the `crate-jmx-exporter-1.2.2.jar` with the CrateDB distribution packages and its OCI image already, similar like `crash` and `admin-ui` are also integrated into the software distribution, to save the manual invocation of `wget`.

b) Provide a little more concise environment variable for activation, instead of needing to manipulate the full `CRATE_JAVA_OPTS`, like currently outlined in the documention.

I am primarily looking at making the invocation of CrateDB in container environments much easier, optimally like:
```shell
docker run --env JMX_EXPORTER=true docker.io/crate
```

/cc @WalBeh