[FEATURE] API Reference for Plugins

[huyaboo]

Is your feature request related to a problem?
I have read the docs as linked in #171, but there is no mention of support for plugins for the OpenSearch bundle (e.g. REST calls for creating an anomaly detector).

What solution would you like?
Either documentation for plugins or Python client support for existing plugins, if such a feature did not yet exist.

What alternatives have you considered?
An alternative would be to manually call the API, but that would take more effort than using the Python client.

Do you have any additional context?
If you could direct me to the plugin documentation for the Python client, that would be much appreciated.

[dblock]

#90 is related

[dblock]

Closing as a dup of #90. We've been adding plugins (see https://github.com/opensearch-project/opensearch-py/blob/main/USER_GUIDE.md#plugins), currently have ISM, security, k-nn and alerting.