DEV: Add API docs for uploads and API doc watcher (#15387)

This commit adds API documentation for the new upload
endpoints related to direct + multipart external uploads.

Also included is a rake task which watches the files in
the spec/requests/api directory and calls a script file
(spec/regenerate_swagger_docs) whenever one changes. This
script runs rake rswag:specs:swaggerize and then copies
the openapi.yml file over to the discourse_api_docs repo
directory, and hits a script there to convert the YML to
JSON so the API docs are refreshed while the server is
still running. This makes the loop of making a doc change
and seeing it in the local server much faster.

The rake task is rake autospec:swagger

spec/swagger_helper.rb

@@ -74,6 +74,18 @@ def api_docs_description
   HEREDOC
 end
 
+def direct_uploads_disclaimer
+  <<~HEREDOC
+    You must have the correct permissions and CORS settings configured in your
+    external provider. We support AWS S3 as the default. See:
+
+    https://meta.discourse.org/t/-/210469#s3-multipart-direct-uploads-4.
+
+    An external file store must be set up and `enable_direct_s3_uploads` must
+    be set to true for this endpoint to function.
+  HEREDOC
+end
+
 RSpec.configure do |config|
   # Specify a root folder where Swagger JSON files are generated
   # NOTE: If you're using the rswag-api to serve API descriptions, you'll need