Support custom link format

A couple of issues with dox stem from incorrect type and field resolution. (#252, #203, possibly also #241 and #184?). The current resolver for links (https://github.com/HaxeFoundation/dox/blob/master/src/dox/MarkdownHandler.hx#L45) could be smarter – check what module a type is in, check the imports, prefer to link to platform-agnostic types, etc etc.

I suggest we instead use a custom link format to link to types and fields properly. Any new docs would / should be written with:

- `` - to link to a package
- `` - to link to a type
- `` - to link to a subtype
- `` to link to a field (variable, property, method)
- possibly other special syntax

This is not a complicated change for the resolver. We can keep the old docs as they are or perhaps systematically find all links and make sure they are correct (a bit time consuming but once done it is done).