Pull md files from doc source repo instead of wiki

[AlanDrake]

Preliminary step to #242:

• Linking to another repo instead of the wiki.
• Added an edit link to github above the footer.

Notes:

• links from github preview don't work because it expects a .md extension
• since the source is still pulled through a submodule, we still have the same pro/con as before that it checks out the same submodule version which was committed

[ericoporto]

links from github preview don't work because it expects a .md extension

I can't understand what this line means...What is GitHub Preview?

[AlanDrake]

What is GitHub Preview?

The preview from the normal file editor. It's almost the same to the wiki editor, but they treat links differently.