[Feature Request]: Add date param for serverless releases

### Prerequisites

- [x] I have searched existing issues to ensure this feature hasn't already been requested
- [x] I have tested using the latest version of docs-builder

### What problem are you trying to solve?

When there's a docs update that applies to serverless, in order to maximize its accuracy we have to time our docs update with the serverless release. For example if a feature will be released next week as GA, I can't merge it this week without it being inaccurate until the release. This is different from how it works for stack releases (where we can specify the version) this is particularly inconvenient because the same PR typically updates the docs for both stack and serverless.

### Proposed Solution

What if we could add a date to the serverless tag, so it could be e.g. `serverless: ga 01/20/2026`, that way, before the given date, it could render as `serverless: planned`, similar to how we can display `stack: planned`. This would enable us to merge a PR before the related serverless release, reducing how much attention we would have to spend tracking release schedules, and create a more consistent display format for Stack and Serverless version support info.

### Examples and Research

_No response_

### Alternative Solutions

_No response_

### Additional Context

_No response_

### How important is this feature to you?

Important