Add info banner for supported but non-latest documentation versions

[kehach07]

Refs #1122 — Docs for old supported versions should indicate that a newer version is available

Adds a small informational banner to documentation pages when viewing a supported but non-default Django version, letting users know a newer version exists and linking to the latest docs. The banner is subtle and does not affect unsupported, preview, or development version warnings.

[adamzap]

@kehach07 Could you provide a screenshot for this change?

[kehach07]

@kehach07 Could you provide a screenshot for this change?

Thanks for the review! @adamzap,

Locally I only have one supported docs version (6.0), so it is treated as the
default and the banner does not normally appear. I temporarily relaxed the
condition to verify the UI and capture this screenshot.

In production, where multiple supported versions exist, the original condition
(release.is_supported and not release.is_default) works as intended.