Previously these were only linked in a circuitous way rather than being
available from the top level API documentation and "Put Lifecycle" API docs.
This makes them slightly easier to find for a user.
* [DOCS] ILM API Ref edits
* [DOCS] Fixed endpoint for DELETE policy.
* [DOCS] Removed comparison to setting index.lifecycle.name to null.
* [DOCS] Fixed xrefs to explain API.
This PR renames the CRUD APIS for ILM
GET _ilm/<policy>, _ilm -> _ilm/policy/<policy>, _ilm/policy
PUT _ilm/<policy> -> _ilm/policy/<policy>
DELETE _ilm/<policy> -> _ilm/policy/<policy>
closes#34929.