Quick cleanup: relative links

[SeanKilleen]

We could do a quick find/replace for links in the docs that start with docs.nunit.org because those are actually best represented as a ~/ style link in docfx to the md file itself. Docfx converts to the correct relative link at build time, and then we get warnings in case a file moves or a link would be broken.

Should be a small find/replace exercise; I don't think there are too many out there.

[SeanKilleen]

Adding this because I noticed while reviewing #606,