Adds a CC-by license for documentation and MIT for code snippets

[dpshelio]

This is adapted from how Carpentries does it for their training material.

This file doesn't introduce any changes in the built documentation (therefore the © notice at the end of the page is untouched - providing a license in the repository doesn't conflict with it).

Provides a fix for #133, but maybe the team prefers a different type of license.