[Feature Request] Allow use of reference links in Markdown

[paveloom]

Just ran into the fact that reference links are not supported: link labels are simply printed as text.

[mortenpi]

This is an upstream issue: the Julia Markdown parser does not support reference links (JuliaLang/julia#19844). There are a few other WIP ideas to work around this:

1. @url #1351 would allow sort of reference links, just with a custom Documenter syntax
2. The other option is to support alternative Markdown parsers (e.g. CommonMark.jl). There is a little bit of WIP on the Documenter side: Integrating Documenter with parsers for non-markdown documents #1074 (comment)