Document how to load Druid extensions

[Jimvin]

The documentation does not contain instructions on how to update the list of extensions that Druid should load upon startup. This can be done at the moment using configOverrides and adding the desired list of extensions to runtime.properties for each role.

An example of how this is done can be found in the OIDC kuttl test - https://github.com/stackabletech/druid-operator/blob/79b3eb38ed680b2a72ae63d935f36f84904957e1/tests/templates/kuttl/oidc/30-druid.yaml.j2

[fhennig]

Yeah it's annoying to do by hand, because you also need to list all the default extensions. I think it would be good to have an extra field for this, pinging @soenkeliebau because he worked on it before, maybe we can prioritize this higher? Should be easy to implement

[soenkeliebau]

I opened #416 but there were some discussions which I do not remember otomh..

[fhennig]

I created this larger epic about extensions in druid, which includes better default selection of extensions, better docs and better config ergonomics: #490

The epic incorporates this ticket as well as the other PR

[fhennig]

This has now been documented: https://docs.stackable.tech/home/nightly/druid/usage-guide/extensions