Add WTC and CTC documentation page (#771)

[vahid-ahmadi]

Summary

• Ticks off the WTC and CTC entries in Documentation pages #771's docs-pages todo list with a single combined page, since the two tax credits share the means-test machinery and were jointly closed to new awards on 5 April 2025.
• New page at docs/book/programs/gov/dwp/tax-credits.md covering:
  • the gov.dwp.tax_credits.active flag that short-circuits the whole computation from 2025-04-06,
  • the WTC and CTC element trees (with pointers to each YAML file),
  • the WTC hours test (min_hours) and the CTC two-child limit + qualifying-child tests,
  • the shared income threshold / reduction-rate / min_benefit floor means-test logic encoded in tax_credits_reduction,
  • take-up via would_claim_WTC / would_claim_CTC,
  • the managed migration to Universal Credit and the post-2025 reported-only state.
• References: Tax Credits Act 2002, SI 2002/2005, SI 2002/2007, HMRC end-of-scheme announcement, CBP-9984 migration briefing, HMRC tax credits statistics page.

Test plan

• Markdown lints cleanly (tables, code blocks, MyST {note} admonition).
• Docs build job picks up the new page under programs/gov/dwp/.

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