Add Edit on Github button (rebased onto develop)



This is the same as gh-800 but rebased onto develop.

----

This PR makes use an Sphinx extension to extend the `This page` sidebar menu (for HTML output only) of all the documentation sets and add an `Edit on GitHub` hyperlink.

To test this PR, 
- look at the staging documentation(s)
- check the `This page` sidebar menu has extra links under the `Show source` 
- check that these links point to the right source file on GitHub