[ES|QL] General improvements

[florent-leborgne]

The ES|QL docs are rapidly growing and require some attention to improve the overall UX:

• Relocate some content into the reference docs
• Assess and improve navigation paths between narrative and reference docs
• Update Get started and subsequent pages in narrative content to improve the learning journey about
  • the language itself
  • each interface that supports the language (incl. Kibana features)
• Review the usefulness and depth of the examples as more commands and functions are added to the language

cc @leemthompo as we spoke about this

[stratoula]

Can we add some docs for ES|QL controls too? They are much more improved in 9.1 cc @ghudgins

[florent-leborgne]

ES|QL controls are documented here: https://www.elastic.co/docs/explore-analyze/dashboards/add-controls#add-esql-control
And I'll edit these docs for 9.1 as part of #1679