docs: add RFC 9421 request signing guide

.changeset/docs-rfc9421-signing-guide.md

@@ -0,0 +1,4 @@
+---
+---
+
+Add RFC 9421 request signing guide at `docs/building/by-layer/L1/request-signing.mdx`. Covers key generation, JWKS/brand.json publication, buyer-side signing, seller-side verification with `requireAuthenticatedOrSigned` + `mcpToolNameResolver`, webhook signing, key rotation, capability declaration, and conformance testing (39 vectors: 12 positive, 27 negative). Code examples in **JavaScript/TypeScript**, **Python** (`adcp.signing`), and **Go** (`adcp-go/adcp/signing`) tabs across all major steps. Storage section covers cloud KMS as the preferred option for spend-committing agents. Adds the page to the L1 nav in `docs.json` and a redirect from the legacy `/docs/building/implementation/request-signing` path. Ported from adcontextprotocol/adcp-client#914.

docs.json

@@ -158,6 +158,7 @@
                         "pages": [
                           "docs/building/by-layer/L1/index",
                           "docs/building/by-layer/L1/security",
+                          "docs/building/by-layer/L1/request-signing",
                           "docs/building/by-layer/L1/webhook-verifier-tuning"
                         ]
                       },
@@ -728,6 +729,7 @@
                     "pages": [
                       "docs/building/by-layer/L1/index",
                       "docs/building/by-layer/L1/security",
+                      "docs/building/by-layer/L1/request-signing",
                       "docs/building/by-layer/L1/webhook-verifier-tuning"
                     ]
                   },
@@ -1572,6 +1574,10 @@
       "source": "/docs/building/implementation/webhook-verifier-tuning",
       "destination": "/docs/building/by-layer/L1/webhook-verifier-tuning"
     },
+    {
+      "source": "/docs/building/implementation/request-signing",
+      "destination": "/docs/building/by-layer/L1/request-signing"
+    },
     {
       "source": "/docs/building/implementation/mcp-response-extraction",
       "destination": "/docs/building/by-layer/L0/mcp-response-extraction"