Add documentation and handlebars validation for helpers

[andrewstucki]

We just added the first handlebars helper for use in packages in elastic/kibana#72698 in order to support some beats filebeat module conversion to packages. We should have:

1. documentation for the available helpers here for anyone writing a new package, and
2. code for validating the handlebars templates prior to being pushed down to Kibana.

[ruflin]

@ycombinator I would consider something like above to be part of the package spec. WDYT?

[ycombinator]

Agreed. Let's move this issue to the package spec repo so we can make sure this work is done there.

[jsoriano]

code for validating the handlebars templates prior to being pushed down to Kibana.

This could have helped detecting elastic/integrations#14674 earlier.