Banners across versions of docs

[victoriaplummer]

Problem description

Currently, there is no built-in functionality in Fern Docs to display banners that that are unique to documentation versions (e.g., version-specific banners for deprecations or critical announcements). Without this feature, maintaining version-specific messaging (like deprecation notices) across different versions of documentation becomes cumbersome and error-prone.

Why would it be useful?

• Providing an easy way to display important announcements, such as API version deprecations, upgrades, or migration reminders, across a single or multiple versions of the documentation.
• Ensuring targeted messaging for specific API users
• Reducing the need for workarounds, custom code, or manual efforts to communicate critical updates effectively.

• Describe the solution (optional)

Expanded, version-specific capabilities for the announcement banner

[dannysheridan]

Thanks for taking the time to write this up, @victoriaplummer! Will follow up when available.