Support mapping links to full URLs from hint GitHub repo

[antross]

What the website is doing is getting these type of links:

/* This regex should match the following examples:
     * * [link label]: ./somewhere.md → \s
     * * [link label]:./somewhere.md → :
     * * [link](./somewhere.md)  → \(
*/

And converting them to a "local" website.

If we add a full URL, for know it will going to keep it as is (IIRC). So the link instead of keeping inside the website, will go to the github documentation.

I think we need to add another rule to process urls that points to the hint repo.

Originally posted by @sarvaje in webhintio/hint#3937 (comment)

[antross]

This will support related changes to make these links work on MDN.

E.g. when the following change to links is made in the linked PR

-[is-valid]: ./docs/is-valid.md
+[is-valid]: https://github.com/webhintio/hint/blob/HEAD/packages/hint-babel-config/docs/is-valid.md