Reconcile version dropdown behavior between /guide and /docs

[KOTungseth]

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.