Add frontmatter URL pointing to markdown source in github

[localredhead]

To allow for our site generator to provide links to the markdown source, it will be necessary to frontload this information by adding it to the frontmatter of each markdown document.

---
title: 
index:
type:
category:
url: <http://......>  
---

For the name of the frontmatter attribute, perhaps url is not descriptive enough, something simple like github instead may be a better option. Work with @gbugyis to determine the most appropriate per the graphQL queries he will be writing to utilize this new frontmatter.

[localredhead]

cc @pelle requested this one.