Each page should link to its source code on GitHub

[noteed]

Whenever you visit a page on this project, it should be super-easy to navigate to its source form on GitHub. To do so, each page should provide a link to its source. There is an example commit for the homepage: ff7eff6

The example should be generalized, possibly with some CPP or Template Haskell magic to auto-generate the right path.

Currently, we don't have any Markdown file, but if we do, they also should link to GitHub.

See also #3