* Reference documentation for rank evaluation API
This adds a first page of reference documentation to the current state of the
rank evaluation API.
Closes to #21402
* Add default values for precision metric.
Add information on default relevant_rating_threshold and ignore_unlabeled
settings.
Relates to #21304
* Move under search request docs, fix formatting
Also removes some detail where it seemed unneeded for reference docs
Isabel Drost-Fromm