The setting `plugin.types` is currently used to load plugins from the
classpath. This is necessary in tests, as well as the transport client.
This change removes the setting, and replaces it with the ability to
directly add plugins when building a transport client, as well as
infrastructure in the integration tests to specify which plugin classes
should be loaded on each node.
* Centralised plugin docs in docs/plugins/
* Moved integrations into same docs
* Moved community clients into the clients section of the docs
* Removed docs/community
Closes#11734Closes#11724Closes#11636Closes#11635Closes#11632Closes#11630Closes#12046Closes#12438Closes#12579