Add knocking to the spec

Spec for https://github.com/matrix-org/matrix-doc/pull/2998
Spec for https://github.com/matrix-org/matrix-doc/pull/2403

This deliberately does not help towards fixing https://github.com/matrix-org/matrix-doc/issues/3153 in order to remain consistent with prior room versions, and to keep the diff smaller on this change. A future change will address room version legibility.

data/event-schemas/examples/knock_room_state.json

@@ -0,0 +1,18 @@
+[
+    {
+        "type": "m.room.name",
+        "sender": "@bob:example.org",
+        "state_key": "",
+        "content": {
+            "name": "Example Room"
+        }
+    },
+    {
+        "type": "m.room.join_rules",
+        "sender": "@bob:example.org",
+        "state_key": "",
+        "content": {
+            "join_rule": "knock"
+        }
+    }
+]

data/event-schemas/examples/m.room.member$knock_room_state.yaml

@@ -0,0 +1,15 @@
+{
+  "$ref": "m.room.member.yaml",
+  "content": {
+    "membership": "knock",
+    "avatar_url": "mxc://example.org/SEsfnsuifSDFSSEF",
+    "displayname": "Alice Margatroid",
+    "reason": "Looking for support"
+  },
+  "unsigned": {
+    "age": 1234,
+    "knock_room_state": {
+      "$ref": "knock_room_state.json"
+    }
+  }
+}