Add an "edit page on GitHub" button

[mithro]

It would be nice if the website had an "edit on GitHub" style button which makes it quick and easy for people to send small changes to the website.

[mithro]

This is kind of a dup of #124

[urish]

#124 actually talks about updating project documentation.

I'm not sure we want people to directly edit the website, usually the quality of the edits we're seeing from random people is not great, to say the least.

Maybe we should direct them to open an issue instead?