docs: add TMP provider implementation guide

[bokelley]

Closes #1733

Adds a step-by-step implementation guide for TMP providers at docs/building/implementation/tmp-provider.mdx. TMP has comprehensive reference and concept docs (docs/trusted-match/) but no implementer guide — this fills the gap.

The guide covers:

• What endpoints a TMP provider exposes (POST /context, POST /identity) and the routing model
• Context Match: request format, response with offers, GAM signals, latency budget (50ms default)
• Identity Match: request format, eligibility response, TTL caching contract, request-ID isolation
• Provider registration schema (context-only, identity-only, full provider, property scoping)
• Health endpoint and provider lifecycle states
• Local testing with the adcp-go reference router
• Surface-specific considerations with cross-links to all five existing surface guides

Registered in docs.json navigation after seller-integration (the analogous sell-side guide) in both navigation instances.

Non-breaking justification: adds one new docs page and two nav entries; no existing file is modified; no schema changes.

Pre-PR review:

• code-reviewer: approved after two blockers fixed — identity request example corrected from type/value to uid_type/user_token per schema; signals example corrected from targeting: {} to targeting_kvs: [{key, value}] per schema; testing section rewritten to remove incorrect adcp check-network usage
• docs-expert: approved — all cross-links resolve, TMP schema foundation complete, additive-only, guide follows seller-integration.mdx pattern

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_0186jsjnPdKzGKZWKrzEKfDf

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