Document future-date policy announcements (#636)

[vahid-ahmadi]

Summary

• Future-date today's policy announcements #636 (filed in 2022, motivated by the Truss "growth plan" tax cuts) asked for an explicit way to encode announced-but-not-yet-implemented policy changes.
• The feature has been live the whole time — date-keyed parameter values are the underlying mechanism and the repo is full of working examples (NI Levy then-cut, Budget 2025 two-child limit removal, PA freeze extension) — but it wasn't documented as a pattern.
• New docs page at docs/book/policy/future-date-announcements.md covering:
  • the mechanism (parameter values keyed by effective date, latest on-or-before query date wins),
  • three common patterns: announced-then-reversed (Truss mini-budget), phased-in changes, announced abolitions/sunsets,
  • best practices: cite every future-dated row, don't extrapolate beyond what was announced, use policy_date queries for back-cast scenarios,
  • distinction from gov.contrib.* reform parameters (uncertain / hypothetical) vs baseline future-dated rows (announced policy).
• Cross-links to the worked example in child-poverty-tcl.md (Validate PolicyEngine child poverty estimates against RF/OBR benchmarks #1398) and the Truss-era blog post.
• Pure documentation; no code changes.

Test plan

• Markdown lints cleanly.
• Docs build job picks up the new page under policy/.

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