2276 feature extract doc (#2277)
2276 feature extract doc (#2277)

* Create feature_extraction.md

* add config documentation

* add use case to the feature extract

* add a remark that fieldsa re not extracted across all events

* Update docs/guides/analyzers/feature_extraction.md

Co-authored-by: Johan Berggren <75f2ca0a64d494ddbb1c410a8e262a526f85898e@gmail.com>
google/timesketch,google/timesketch,google/timesketch,google/timesketch