Fix paths to markdeep in documentation templates

[Caid11]

I noticed this error when I tried to use the builtin documentation templates:

.../todo.md.html:17  GET http://127.0.0.1:8000/doc/markdeep.min.js net::ERR_ABORTED 403 (Illegal webpath (731): /doc/markdeep.min.js)

I switched the markdeep path from /doc/markdeep.min.js to /src/quadplay/doc/markdeep.min.js (the same path used by the manual), and that fixed the error.

Also, I saw a second error related to markdeep:

todo.md.html:17  GET https://casual-effects.com/markdeep/latest/markdeep.min.js? net::ERR_BLOCKED_BY_RESPONSE.NotSameOriginAfterDefaultedToSameOriginByCoep 200 (OK)

This looks like some sort of CORS error. I'm not very familiar with CORS, so I didn't try to fix that issue.