Add `Edit on GitHub` buttons to every page

[LiNk-NY]

It would be great to have an Edit on GitHub button on each page so that users can contribute to the content of the website.

It can be a button or link and it can look like this:

Edit this page

where it points to https://github.com/Bioconductor/bioconductor.org/edit/master/content/developers/package-guidelines.md