Adds documentation for the EQL `substring` function.
Supporting changes:
* Creates a new "EQL function reference" page
* Updates the title of the "EQL syntax reference" page for consistency
* Adds a brief "Functions" section to the EQL syntax docs
* Updates EQL limitations docs to state that only array functions are
unsupported
Adds documentation for the `any` keyword to the EQL syntax docs.
Includes:
* Definition of an event category and its relationship to the event
category field.
* Example matching all event categories using `any` keyword
* Example using `any` with `where true`
Adds documentation for basic EQL syntax.
Joins, sequences, and other syntax to be added as its supported
in future development.
Co-Authored-By: Ross Wolf <31489089+rw-access@users.noreply.github.com>
James Rodewig
Aleksandr Maus