Banners: Support start/end dates for announcement banners

[CamSoper]

Problem

The sitewide announcement banner is configured in data/announcements.yml. Today an entry is shown or hidden via a manual active: true|false toggle. There is no way to schedule a banner to turn off (or on) automatically.

This bit us during launch week: the launch-week banner stayed up after the campaign ended because someone had to remember to flip it off. It took a manual PR (#19542) to take it down, and in the meantime the stale banner lingered on the blog (with a side of caching confusion).

Request

Add optional scheduling fields to each announcement entry so banners activate and expire on their own, e.g.:

announcements:
  - id: blog-agentic-infrastructure-era-2026-05-Kp4nQ2
    active: true
    start: 2026-06-01   # optional — show on/after this date
    end: 2026-06-05     # optional — stop showing after this date
    display: both
    ...

Suggested behavior:

• start and end are both optional. Omitting start means "active immediately"; omitting end means "no expiry" (current behavior).
• An entry renders only when it is active: true and the current date is within the [start, end] window.
• Keep the existing active flag as a hard master switch so an entry can be force-disabled regardless of dates.
• Decide and document the timezone semantics (UTC is probably simplest) and whether end is inclusive.
• Update the header comment in data/announcements.yml to document the new fields.

Why

• No more stale banners haunting the blog after a campaign ends.
• No need to remember to cut a takedown PR — schedule it once when the banner goes up.

Context

Requested in Slack by Tatiana Cooke, with a +1 from Pablo Seibelt ("It'd be nice to be able to set them to have start/end date instead so we don't need to remember to take it down"). Filed at Cam Soper's request.

Slack thread: https://pulumi.slack.com/archives/CCBFCGU94/p1780698059160009