feat: Markdown parser to handle cdn /dev and /deploy assets

[darcywong00]

Background info
keymanapp/help.keyman.com#2347

Currently, guides in Markdown can reference image assets in the /cdn/ folder.
When the site is built with the --debug flag, /cdn/deploy assets are not generated

https://github.com/keymanapp/help.keyman.com/blob/2eeb860a21c050d5c26e126841fee105d27ef1a2/resources/init-container.sh#L11-L20

From @mcdurdin

Limitation of the markdown files at present. We should be using cdn/dev and then the markdown parser should be rewriting them, ideally.

[mcdurdin]

See also keymanapp/help.keyman.com#2348