chore: reconcile top-level header sizes with the mdBook navbar parser

Author: nekename

src/README.md

@@ -1,4 +1,4 @@
-## OpenAction
+# OpenAction
 
 ### Welcome!
 

src/api/registration.md

@@ -1,4 +1,4 @@
-## Registration
+# Registration
 
 The OpenAction server will start your plugin with arguments specifying the means of initialising the WebSocket connection. If you use a plugin SDK, such as openaction-rs, you may be able to skip these steps.
 

src/manifest.md

@@ -1,4 +1,4 @@
-## Manifest
+# Manifest
 
 Your plugin needs to contain a `manifest.json` file that supplies key information about your plugin to the OpenAction server. It should be present in the root of your plugin directory. The file is the primary description of your plugin and contains information like the path to your plugin's entrypoint and the actions it provides.
 

src/overview.md

@@ -1,4 +1,4 @@
-## Overview
+# Overview
 
 OpenAction is an event-driven API that communicates over WebSocket.