[DOC] UX improvements in "Create threat detector" flow in Security Analytics

[xeniatup]

What do you want to do?

• Request a change to existing documentation

https://opensearch.org/docs/latest/security-analytics/sec-analytics-config/detectors-config/

Tell us about your request.
We're making changes in "Create detector" flow:

1. The third step (review page) is removed
2. There are changes in wording (automatically mapped fields = mapped fields, pending field mappings = available fields, etc.)
3. There are changes in the page hierarchy so an update to the screenshots will be required
4. The "Alert trigger" step is redesigned.

What other resources are available?
UX issue with the changes described in more detail:
opensearch-project/security-analytics-dashboards-plugin#740