Change source URL for Markdeep

[hollasch]

Looks like the preferred URL for markdeep.min.js has changed from https://casual-effects.com/markdeep/laetst/markdeep.min.js to https://morgan3d.github.io/markdeep/latest/markdeep.min.js. We should update the books and the acknowledgements.

[hollasch]

Done.

[trevordblack]

This needs to be ported v4

The comment at the top of the books:

<!-- Markdeep: https://casual-effects.com/markdeep/ -->

needs to also be updated

[hollasch]

Porting happens automatically at patch release (I typically merge to more major releases shortly afterward).

The URL is fine — I updated the URL for the release point of the Markdeep library, but the main documentation site remains on casual-effects.com.