Add an "edit this page" button that leads to the right class on GitHub

[philipsens]

Frank!Developers should be encouraged to improve the Frank!Doc content if they encounter something that isn't clear.

Adding a link would help, so people get the feeling that it is easy and a possibility to edit the docs themselves. It would be easier because they don't have to search fore the right file.

It could even link to the "edit in place" page in GitHub, like this: https://github.com/ibissource/iaf/edit/master/core/src/main/java/nl/nn/adapterframework/pipes/Json2XmlValidator.java