docs(building): add schemas-and-sdks orientation page

[garvitkaushik-123]

Summary

Closes #5290.

The path /docs/building/schemas-and-sdks is referenced from 10+ pages across the docs (intro.mdx, versioning.mdx, learning tracks, refinement guide, etc.) but the file never existed — every link was a 404.

This adds a thin orientation page covering:

• SDK install quickstart — npm/pip/go one-liners for all three official SDKs
• Schema URL conventions — versioned vs. latest URL patterns with examples
• Authorization types — brief overview of the 6 adagents.json authorization types with a cross-link to the authoritative Authorization Patterns reference (no duplication)
• Where to go next — card links to choose-your-sdk, build-an-agent, build-a-caller, validate-your-agent

Per the triage: this is a cross-link hub, not a duplication of existing content. The detailed coverage matrix lives in choose-your-sdk.mdx, the layer model in sdk-stack.mdx, and the full authorization reference in adagents.mdx.

Test plan

• Verify the page renders in Mintlify local preview
• Confirm existing links from intro.mdx, versioning.mdx, platform.mdx, buyer.mdx, and refinement.mdx now resolve instead of 404ing
• Check all outbound card links resolve

🤖 Generated with Claude Code

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[aao-release-bot]

Closes the 404. Right call to make this a thin cross-link hub instead of re-duplicating choose-your-sdk / sdk-stack / adagents — the detail lives in one place and this page just routes to it.

Lineage checks out: PR #4031 (IA Phase 3) split the original schemas-and-sdks.mdx out of source, which is why 8 pages (intro.mdx, versioning.mdx, buyer.mdx, platform.mdx, refinement.mdx, release-notes.mdx, migration/index.mdx, verifying-protocol-tarballs.mdx) were pointing at a path that no longer existed in latest. The dist snapshots still carry the old 339-line frozen copy, so the page only 404'd against current source. This re-adds it.

Things I checked

• All six adagents.json authorization types named on the page — property_ids, property_tags, inline_properties, publisher_properties, signal_ids, signal_tags — match the discriminator enum in docs/governance/property/adagents.mdx:266 and the Authorization Patterns table at adagents.mdx:483-488. No drift.
• The #authorization-patterns anchor the page cross-links to exists (adagents.mdx:477).
• Every outbound card/inline link resolves: choose-your-sdk, build-an-agent, build-a-caller, sdk-stack, validate-your-agent, versioning, adagents.
• Schema URL example https://adcontextprotocol.org/schemas/v3/media-buy/create-media-buy-request.json is a real published path — the non-bundled $ref variant exists alongside the bundled/ form in the schema tree. v3/ major alias matches the convention already documented in by-layer/L0/schemas.mdx:119.
• docs.json parses as valid JSON; the page is wired into all three nav trees and the dist/docs/3.0.19/building/schemas-and-sdks entry points at a file that exists and is tracked.
• Test plan: all three boxes (Mintlify preview, inbound link resolution, outbound card links) are unchecked. The primary fix is link resolution — I verified the 8 inbound references and every outbound target above resolve, so the page ships validated against the path it claims to fix even though the author didn't tick the boxes.

Follow-ups (non-blocking — file as issues)

• The 3.0.19 snapshot nav now surfaces the old 339-line schemas-and-sdks.mdx while latest gets this 63-line hub. That's correct version-frozen behavior, not a defect — but the two pages tell different stories (full coverage vs. cross-link hub). Worth knowing the next snapshot will freeze the thin version forward.

Minor nits (non-blocking)

1. docs.json em-dash churn. The functional change is three inserted nav lines, but the diff also re-encodes every literal — in the file as — (banner, og:site_name, the five L0–L4 group titles, the AI-disclosure footer). Renders identically, so it's harmless — just noise that makes a one-line nav add read as a 21-line edit. An editor re-serialized the file; worth reverting the unrelated lines if it's cheap.

LGTM. Follow-ups noted below.