Set up a way that the tip of a pull request is deployed to github pages

[moorepants]

One way to do this is to make a new directory structure that has:

https://moorepants.github.io/learn-multibody-dynamics/pull-requests/branch-name

Then each pull request could build the version of the website and deploy to a subdirectory. When the pull request is merged, that directory should be deleted on gh-pages.

[moorepants]

I got something mostly working in 2246b1e but the result doesn't display the JS external libs, see: htmlpreview/htmlpreview.github.com#109

This was also suggested:

https://docs.netlify.com/site-deploys/deploy-previews/

[moorepants]

This now works. I forked htmlpreview and applied the pr to let external JS run. There is now an action that manages the commenting and branch creation: #27 the only issue is that the branch isn't deleted on merge. That has to be done manually, but can be fixed later.