[Q2 FY27] Make ES|QL in Kibana more approachable

ES|QL is evolving fast:
- the editor is becoming powerful
- the experience in Discover improves quickly with things such as variables/controls, lookup index editor...
- ES|QL visualizations will soon be a "mode" in Lens rather than its own panel
- ES|QL is powering more and more features "under the hood"
- ES|QL is becoming more present in alerting too

The overarching logic there is that ES|QL is becoming a lot more central to the experience pretty fast. In terms of documentation, this should be reflected, and help users adopt it.

**The docs now**
- individual pages "on the side" for each feature
- a growing list of sections without a unified narrative that is becoming less digestible

**The docs after this issue**
- ES|QL is still not the default mode in the experience, but the narrative is much more integrated into relevant docs page. The main focus is still on current default modes, but the docs are much more balanced and ES|QL is mentioned and explained more often, where needed.
- Better structured and well-placed ES|QL tutorial or getting started pages for relevant features (Discover, Dashboards, alerts, else?)
- The "ES|QL in Kibana" main page is restructured and much more digestible. It references key usage information for anyone looking to start using the editor or having questions about specific 'manoeuvers' with the editor. The page feels less like a big pile of perks.

**The docs in the future (to distinguish what the current task will not achieve but that we can start thinking about)**
- ES|QL will one day be the default experience in some apps, and the docs will reflect that as they typically show default states first.