Add “What you’ll learn” section to reference architecture docs for reader orientation

### Summary
Add a short reader-orientation section (e.g. “What you’ll learn”) to the reference architecture documents, placed after the intro and before the first technical section. This would improve onboarding and give readers a quick overview before diving into details.

### Context
In [this PR](https://github.com/elastic/docs-content/pull/5510) for the GenAI search high-availability reference architecture, reviewers [noted](https://github.com/elastic/docs-content/pull/5510#discussion_r2964788795) that the document jumps quickly into detail with little reader orientation. The suggestion is to add a brief section (something like “What you’ll learn”) that outlines the main topics, such as:

Use cases
Architecture
Hardware spec
Considerations

This would act as both an overview and a simple index.

For consistency, apply the same pattern to both reference architecture documents so they stay aligned and maintain continuity.