docs: add post editor demo video

Convert the post editor guide to MDX so the embedded YouTube walkthrough renders correctly and give the page a quick visual demo near the top.

Author: AvdLee

…s/docs/features/capturing/post-editor.md …/docs/features/capturing/post-editor.mdxdocs/src/content/docs/docs/features/capturing/post-editor.md renamed to docs/src/content/docs/docs/features/capturing/post-editor.mdx docs/src/content/docs/docs/features/capturing/post-editor.md renamed to docs/src/content/docs/docs/features/capturing/post-editor.mdx

@@ -9,6 +9,15 @@ RocketSim 15 adds a built-in post editor for both screenshots and recordings. Af
 
 RocketSim refers to this window as the **Capture Editor** in the app, but you can think of it as the post-editing step for your captures.
 
+## Watch the post editor in action
+
+If you want a quick walkthrough before diving into the details below, this demo video shows the post editor workflow for captured media.
+
+<Youtube
+  url="https://www.youtube.com/watch?v=3LlGj6oVi7A"
+  title="RocketSim Post Editor demo"
+/>
+
 ## Opening the editor
 
 The quickest way to open the post editor is from the [floating thumbnail](/docs/features/capturing/floating-thumbnail) that appears after you create a screenshot or recording.