Fix relative to absolute url conversion

[ryanfowler]

This PR removes the custom path joining for path relative URLs and relies on the url.ResolveReference() method to convert a path relative URL to an absolute URL.

In the current main branch, all images from this page are broken after parsing.

With a root URL of https://martinfowler.com/articles/patterns-of-distributed-systems/paxos.html, the image <img src="mfpaxos-initial-requests.png"> is currently converted to:

https://martinfowler.com/articles/patterns-of-distributed-systems/paxos.html/mfpaxos-initial-requests.png

when it should be converted to:

https://martinfowler.com/articles/patterns-of-distributed-systems/mfpaxos-initial-requests.png