Add edit link to the bottom of every Learn Semantic article

[mnquintana]

This idea is very much inspired by what npm did with docs.npmjs.com – we should add a dynamically generated "edit this article" link to the bottom of every article on Learn Semantic. This would drastically lower the barrier of entry for people contributing to these articles – instead of having to shift context away to GitHub to hunt for the article they were just reading on Learn Semantic, they can go straight to editing it.

See https://docs.npmjs.com/getting-started/what-is-npm for example:

Take a look at this template and this script to see how they did it.

[jlukic]

Will add