Changes:
* Moves `Retrieve selected fields` to its own page and adds a title abbreviation.
* Adds existing script and stored fields content to `Retrieve selected fields`
* Adds a xref for `Retrieve selected fields` to `Search your data`
* Adds related redirects and updates existing xrefs
This change explains why Painless doesn't natively support datetime now, and
gives examples of how to create a version of now through user-defined
parameters.
This change abstracts the specific types away from the different
representations of datetime as a datetime representation in code can be all
kinds of different things. This defines the three most common types of
datetimes as numeric, string, and complex while outlining the type most
typically used for these as long, String, and ZonedDateTime, respectively.
Documentation uses the definitions while examples use the types. This makes
the documentation easier to consume especially for people from a non-Java
background.
