Restore and redesign “Most Popular” sidebar

## Description
When we launched Elastic Docs v3, the "Most Popular" sidebar, which is surfaced through the Marketing team, was removed. This section helped users quickly find high-traffic, high-value content and was particularly useful for new users trying to navigate the docs. The current default sidebar is static and does not provide helpful entry points or guidance.


We need to restore a version of the "Most Popular" sidebar with a refreshed design and thoughtful content strategy. This should also consider the future integration of onboarding and getting-started content.

## Goals

- Reintroduce the “Most Popular” or “Trending” sidebar to improve discoverability of key content.
- Provide a curated or data-driven list of frequently visited or essential pages.
- Make space for onboarding-related content in the future (e.g., getting started guides, tutorials, “Start here” links).

## Requirements

- Design exploration for the sidebar (placement, behavior, style)
- Analytics-informed or curated list of top docs
- Mechanism to regularly update or auto-populate the list
- Optional: tag or bucket content for new users, such as “New to Elastic?” or “Get Started”