[Q2 FY27] Restructure Discover docs for better onboarding and goal-oriented tasks

Discover is a very powerful application to explore all data, and has dedicated experiences for the most common / critical types of data such as logs, metrics, traces...
It is essential that users can properly learn how to use it, in both classic and ES|QL mode, and quickly find information for the most common and important actions they can do within the application, and in connection to other applications too, such as dashboards or solution apps.

This is also a great opportunity to adopt best documentation practices with better defined content types and a more discoverable sequence of pages with more targeted scopes.

A couple of months ago I experimented with this, which led to this PR https://github.com/elastic/docs-content/pull/3942.

I intend to resurrect and continue/redo some of this work and work with the Discover team as well as other resources I'll find (Forums, enhancement requests, etc.) to identify gaps and opportunities with this doc.

The product evolution also calls for this re-work as the structure as almost not evolved in years despite numerous features being added or changed, and will keep evolving greatly with the OneDiscover product initiative.

This also relates to #4888