Move raw API and event schemas into /data directory

Historical note: this was originally a series of several commits, spread out
over several weeks. They have been squashed together to make `git annotate`
work properly.

The original commits were:
 * 91ab3934 <Will> 2021-01-25 21:16:42 -0800 Add raw API end event schemas into /data directory
 * aae22f47 <Will> 2021-01-25 21:33:06 -0800 Remove non-data files
 * 1092d4ca <Will> 2021-01-26 20:41:33 -0800 Add data-compatiuble extension (.yaml) to all data files that currently omit one
 * 21060109 <Will> 2021-01-26 20:57:28 -0800 Remove symlink to event-schemas, and update openAPI schema paths accordingly
 * 4f633845 <Travis Ralston> 2021-04-12 21:54:54 -0600 Fix event schema examples too
 * 301c7b2f <Will> 2021-02-05 10:15:42 -0800 Restore docs describing OpenAPI extensions that we use

data/api/server-server/definitions/pdu.yaml

@@ -0,0 +1,53 @@
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+type: object
+title: Persistent Data Unit
+description: A persistent data unit (event) for room versions 1 and 2.
+example:
+  $ref: "../examples/pdu.json"
+allOf:
+  - $ref: "unsigned_pdu.yaml"
+  - type: object
+    properties:
+      hashes:
+        type: object
+        title: Event Hash
+        description: |-
+          Content hashes of the PDU, following the algorithm specified in `Signing Events`_.
+        example: {
+          "sha256": "ThisHashCoversAllFieldsInCaseThisIsRedacted"
+        }
+        properties:
+          sha256:
+            type: string
+            description: The hash.
+            example: ThisHashCoversAllFieldsInCaseThisIsRedacted
+        required: ['sha256']
+      signatures:
+        type: object
+        description: |-
+          Signatures for the PDU, following the algorithm specified in `Signing Events`_.
+        example: {
+          "example.com": {
+            "ed25519:key_version:": "86BytesOfSignatureOfTheRedactedEvent"
+          }
+        }
+        additionalProperties:
+          type: object
+          title: Server Signatures
+          additionalProperties:
+            type: string
+    required:
+      - hashes
+      - signatures