Move API Reference (#23777)

* merge mattermost-api-reference unchanged

* api: update repostiory paths

* api: drop GitPod for api (for now)

* api: improved node_modules target

* api: relocate GitHub actions to root

* Update .github/workflows/api.yml

Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

* fix cache-dependency-path

* adopt node-version-file

* pin versions for uses

* tidy steps/runs

* api/.gitpod.yml: tidy

* api: rm now unused .gitlab-ci.yml

---------

Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

api/v4/source/usage.yaml

@@ -0,0 +1,52 @@
+  /api/v4/usage/posts:
+    get:
+      tags:
+        - usage
+      summary: Get current usage of posts
+      description: >
+        Retrieve rounded off total no. of posts for this instance.
+        Example: returns 4000 instead of 4321
+
+        ##### Permissions
+
+        Must be authenticated.
+
+        __Minimum server version__: 7.0
+      operationId: GetPostsUsage
+      responses:
+        "200":
+          description: Total no. of posts returned successfully
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/PostsUsage"
+        "401":
+          $ref: "#/components/responses/Unauthorized"
+        "500":
+          $ref: "#/components/responses/InternalServerError"
+  /api/v4/usage/storage:
+    get:
+      tags:
+        - usage
+      summary: Get the total file storage usage for the instance in bytes.
+      description: >
+        Get the total file storage usage for the instance in bytes rounded down to the most significant digit.
+        Example: returns 4000 instead of 4321
+
+        ##### Permissions
+
+        Must be authenticated.
+
+        __Minimum server version__: 7.1
+      operationId: GetStorageUsage
+      responses:
+        "200":
+          description: The total file storage usage for the instance in bytes rounded down to the most significant digit.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/StorageUsage"
+        "401":
+          $ref: "#/components/responses/Unauthorized"
+        "500":
+          $ref: "#/components/responses/InternalServerError"