docsite: Add 'Edit on GitHub' for module docs (#36667)

This is something I always wanted, a 'Edit on GitHub' button for module
documentation.

I also removed the additional statement in the footer with instructions
on how to edit the module documentation.

PS The links go directly into the GitHub file editor now !

docs/templates/plugin.rst.j2

@@ -1,3 +1,5 @@
+:source: @{ source }@
+
 .. _@{ module }@:
 
 {% if short_description %}