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/identity/v2_ping.yaml

@@ -0,0 +1,46 @@
+# Copyright 2018 Kamax Sàrl
+# Copyright 2018 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# 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.
+
+swagger: "2.0"
+info:
+  title: "Matrix Identity Service Ping API"
+  version: "2.0.0"
+host: localhost:8090
+schemes:
+  - https
+basePath: /_matrix/identity
+produces:
+  - application/json
+paths:
+  "/v2":
+    get:
+      summary: Checks that an identity server is available at this API endpoint.
+      description: |-
+        Checks that an identity server is available at this API endpoint.
+
+        To discover that an identity server is available at a specific URL,
+        this endpoint can be queried and will return an empty object.
+
+        This is primarly used for auto-discovery and health check purposes
+        by entities acting as a client for the identity server.
+      operationId: pingV2
+      responses:
+        200:
+          description: An identity server is ready to serve requests.
+          examples:
+            application/json: {}
+          schema:
+            type: object