Reconcile version dropdown behavior between /guide and /docs

## Description
The version dropdown experience differs between the /guide and /docs docs. We need to align the behavior and presentation to ensure a consistent user experience across both Elastic Docs sites.

## Current behavior

In /guide:

- The version dropdown appears on the left, above the navigation.
- Users can select different versions within the 8.x series and earlier.
- Versions 9.0 and later are not included in the dropdown.
- Instead, a link to the equivalent page in /docs (where 9.0+ docs live) appears at the top of every page.

In /docs:

- The version dropdown appears on the right, above the “On this page” content.
- Users can select any of the /guide docs in versions 7.19 and the 8.x series.

## Problem

- The placement and functionality of the version dropdown differ between /guide and /docs.
- This is confusing users navigating between legacy (/guide) and current (/docs) docs.

## Proposal

- Decide on a unified experience for version selection across /guide and /docs.
- Determine whether the dropdown should appear on the left or right for both locations. Most docs sites put it on the left.
- Define consistent behavior for how 9.0+ versions are represented (dropdown inclusion vs. link).

## Acceptance criteria

- A consistent location and behavior for the version dropdown across /guide and /docs.
- Clear logic for how users navigate to 9.0+ versions (either through dropdown or top-of-page link).
- Updated implementation and documentation to reflect the chosen design.