docs(reference): re-apply whats-new-in-3-1.mdx (post-revert restore)

.changeset/whats-new-in-3-1-restore.md

@@ -0,0 +1,9 @@
+---
+"adcontextprotocol": patch
+---
+
+Re-apply `docs/reference/whats-new-in-3-1.mdx` cleanly from current main after the original landing (PR #4784) was reverted to restore accidentally-deleted `dist/docs/` versioned snapshots. Content is identical to the original page — comprehensive 3.0 → 3.1 narrative covering 15 headline features synthesized from a full audit of every spec PR merged since 3.0.6.
+
+Adds two nav entries in `docs.json` under the existing **AdCP 3.0** groups.
+
+Closes the gap reopened by the revert.

docs.json

@@ -85,6 +85,7 @@
                     "expanded": false,
                     "pages": [
                       "docs/reference/whats-new-in-v3",
+                      "docs/reference/whats-new-in-3-1",
                       "docs/reference/migration/v3-readiness",
                       "docs/reference/migration/index",
                       "docs/reference/migration/prerelease-upgrades",
@@ -681,6 +682,7 @@
                 "expanded": false,
                 "pages": [
                   "docs/reference/whats-new-in-v3",
+                  "docs/reference/whats-new-in-3-1",
                   "docs/reference/migration/v3-readiness",
                   "docs/reference/migration/index",
                   "docs/reference/migration/prerelease-upgrades",