Add "edit in github" button to docs

[ddavidebor]

The docs should have a "edit in github" button. I found it very hard to contribute to the documentation.

Follows https://discuss.erpnext.com/t/improve-documentation/31247/3

David

[joolsr]

This is in some of the docs but obviously not everywhere

[joolsr]

I think this addresses the issue #4586

[ddavidebor]

Nope, i'm referring to https://erpnext.org/docs

[strixaluco]

Fixed with Improve this page link at the bottom of the page.