Link to edit source of page on GitHub, for pages outside of the docs

[wetneb]

The documentation pages have a link to directly propose changes via GitHub without having to figure out which repository and file generate the docs you see. For instance at https://openrefine.org/docs it looks like this:

It would be nice to have the same sort of functionality for pages of the website which are not part of the docs, such as https://openrefine.org/extensions

[magdmartin]

@antoine2711 following our conversation here is the ticket to add the edit page on any page of the website