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/transactions.yaml

@@ -0,0 +1,109 @@
+# 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.
+
+swagger: '2.0'
+info:
+  title: "Matrix Federation Transaction API"
+  version: "1.0.0"
+host: localhost:8448
+schemes:
+  - https
+basePath: /_matrix/federation/v1
+consumes:
+  - application/json
+produces:
+  - application/json
+securityDefinitions:
+  $ref: definitions/security.yaml
+paths:
+  "/send/{txnId}":
+    put:
+      summary: Send a transaction
+      description: |-
+        Push messages representing live activity to another server. The destination name
+        will be set to that of the receiving server itself. Each embedded PDU in the
+        transaction body will be processed.
+
+        The sending server must wait and retry for a 200 OK response before sending a
+        transaction with a different ``txnId`` to the receiving server.
+
+        Note that events have a different format depending on the room version - check
+        the `room version specification`_ for precise event formats.
+      operationId: sendTransaction
+      security:
+        - signedRequest: []
+      parameters:
+        - in: path
+          name: txnId
+          type: string
+          description: |-
+            The transaction ID.
+          required: true
+          x-example: S0meTransacti0nId
+        - in: body
+          name: body
+          type: object
+          required: true
+          schema:
+            allOf:
+              - $ref: "definitions/transaction.yaml"
+              - type: object
+                properties:
+                  edus:
+                    type: array
+                    description: |-
+                      List of ephemeral messages. May be omitted if there are no ephemeral
+                      messages to be sent. Must not include more than 100 EDUs.
+                    items:
+                      $ref: "definitions/edu.yaml"
+          example: {
+            "$ref": "examples/transaction.json",
+            "edus": [{"$ref": "edu.json"}] # Relative to the examples directory
+          }
+      responses:
+        200:
+          description: |-
+            The result of processing the transaction. The server is to use this response even in
+            the event of one or more PDUs failing to be processed.
+          schema:
+            type: object
+            title: PDU Processing Results
+            description: The results for the processing of each PDU in the transaction.
+            properties:
+              pdus:
+                type: object
+                description: |-
+                  The PDUs from the original transaction. The string key represents the ID of the
+                  PDU (event) that was processed.
+                additionalProperties:
+                  type: object
+                  title: PDU Processing Result
+                  description: Information about how the PDU was handled.
+                  properties:
+                    error:
+                      type: string
+                      description: |-
+                        A human readable description about what went wrong in processing this PDU.
+                        If no error is present, the PDU can be considered successfully handled.
+                      example: "You are not allowed to send a message to this room."
+            required: ['pdus']
+          examples:
+            application/json: {
+              "pdus": {
+                "$successful_event:example.org": {},
+                "$failed_event:example.org": {
+                  "error": "You are not allowed to send a message to this room."
+                }
+              }
+            }