[fix] responsive image support for Markdown Post

[Chen-Gary]

If we add an image to a post without the right sidebar (e.g. the FAQ page in the demo), the image will not resize automatically as the window size changes.

I fix this by adding a Hugo "Markdown render hooks" for markdown image. Reference: https://gohugo.io/templates/render-hooks/#image-markdown-example

The image will render correctly with this file added.

Note: Posts with the right sidebar do render images correctly, even without this render hook (e.g. this post in demo).