ES|QL: make use of FunctionInfo.preview on the documentation side

[astefan]

Description

Functions' FunctionInfo annotation has a preview component for functions that are still in preview/snapshot. But, this is not reflected in how the documentation is generated (if a function is in preview/snapshot-only mode it should be in the docs) nor is how this part is exposed to Kibana. We should look into:

• do we need a snapshot-only component to FunctionInfo or preview() is enough?
• integrate either preview() or a snapshot-only component in the above mentioned places in code.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[luigidellaquila]

Snapshot functions are not in the docs (neither in Kibana JSON docs), so I guess the only information we need is the "preview".

Separately, if we also want Kibana to receive Snapshot functions, we can add another information, but they are two distinct things.