[Docs] Fix Java Api index administration usage (#28133)

[cbuescher]

The Java API documentation for index administration currenty is wrong because
the PutMappingRequestBuilder#setSource(Object... source) an
CreateIndexRequestBuilder#addMapping(String type, Object... source) methods
delegate to methods that check that the input arguments are valid key/value
pairs:

https://www.elastic.co/guide/en/elasticsearch/client/java-api/current/java-admin-indices.html

This changes the docs so the java api code examples are included from
documentation integration tests so we detect compile and runtime issues earlier.

Closes #28131

[cbuescher]

This previously broke the docs build when merged in #28133 because the included tests were not correctly checked out by the docs build script. I moved the test with the included code to a news package under "o.e.client.documentation" in the "server" module now because it doesn't really fit into the high-level client project. This will need a change similar to something like https://github.com/elastic/docs/blob/master/conf.yaml#L204 I think which I will need to add first.

[cbuescher]

Just created the necessary extension of the docs conf.yaml: elastic/docs#300

[martijnvg]

@cbuescher Maybe move the test to ./clien/transport/src/test/java/o.e.client.documentation/ instead of where it is now? Other than that this is looks good.

[cbuescher]

@martijnvg thanks, good point. In the meantime, #28814 already introduced a documentation test class in server/src/test/java/org/elasticsearch/client/documentation which I'd like to extend instead. I wouldn't like moving it elsewhere at this point since the docs build also needs to be aware of this (elastic/docs#316) and it seems to work at least on master already.
I pushed an update which merges the two test classes, would you mind taking another quick look and leave a formal approvement if you think this looks good now?

[martijnvg]

LGTM - Thanks @cbuescher!