Document or improve upgrading 3rd party components (JMX Exporter, SQL Exporter)

In a previously internal issue, @tomach wrote:
> We patched our cloud clusters manually with the new version of jmx-exporter (https://github.com/crate/cloud/issues/2062) but customers using the crate-operator on self-managed clusters are struggling with doing this manually.
>
> We should at least document it in the crate-operator what needs to be done when upgrading from/to which version. And maybe also discuss if we need to add this to the operator to handle it automatically.

This also effects the `SQL_EXPORTER_IMAGE` environment variable. In the existing documentation, the [Configuration](https://crate-operator.readthedocs.io/en/latest/configuration.html#envvar-JMX_EXPORTER_VERSION) page lists `JMX_EXPORTER_VERSION`, but not `SQL_EXPORTER_IMAGE`. The [crate.operator.config module](https://crate-operator.readthedocs.io/en/latest/ref/crate.operator.config.html#crate.operator.config.Config.SQL_EXPORTER_IMAGE) page, however, lists both (?).

Start with the Configuration page and the crate.operator.config.Config.SQL_EXPORTER_IMAGE reference, comparing their coverage of JMX_EXPORTER_VERSION and SQL_EXPORTER_IMAGE. Document the relevant upgrade steps and version guidance for JMX Exporter and SQL Exporter, and clarify whether upgrades are manual or handled by the operator.

Written by the indexing model from the issue text.