[mkdocs] Keep "Edit this page" option

[robertclaus]

Currently we have this:

It would be ideal to keep it on the new site

[daexs]

Button has been added, however, the link it opens is https://github.com/daexs/plotly.py/edit/mkdocs-conversion/pages/... (which also doesn't work due to recent refactoring), we would need to change it to the plotly/plotly.py repo's corresponding branch.