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>