Allowing `_doc` as a type will enable users to make the transition to 7.0
smoother since the index APIs will be `PUT index/_doc/id` and `POST index/_doc`.
This also moves most of the documentation to `_doc` as a type name.
Closes#27750Closes#27751
It adds notes about:
- how preference can help optimize cache usage
- the fact that too many replicas can hurt search performance due to lower
utilization of the filesystem cache
- how index sorting can improve _source compression
- how always putting fields in the same order in documents can improve _source
compression
This moves the "Performance Considerations for Elasticsearch Indexing" blog post
to the reference guide and adds similar recommendations for tuning disk usage
and search speed.
Adrien Grand
Christoph Büscher
Lee Hinman
Lee Hinman
Tanguy Leroux