Additional support for docs for ES|QL operators and version-specific differentiation (elastic#125251)

This PR was originally focused on improving support for Kibana docs, in particular the missing operator docs, but it has expanded to cover a bunch of related things:

* Primarily the main work was to improve operators support. ESQL generated docs cover all functions and most operators for which their is a clear operator class and test class. However, some are built-in behaviour and need additional support. This PR adds more generated content for those operators.
* Various specific operators requested by Kibana: Cast & null-predicates, and in particular the addition of examples
* Two functions without examples: mv_append and to_date_nanos
* Many small visual document cleanups (spelling, grammar, capitalization, etc.)
* Initial support for `applies_to` for multi-version differentiation.

This last point requires more work, as it is not yet agreed on just how we want this to look. We'll probably need to do refinements in followup PR. Consider the version in this PR as a first step into how this could look.

Author: craigtaverner