Add sidebar

[mguerreiro]

This allows one to add a sidebar (menu, infos, ...) without having to fork the whole layout.

This was the best way I found to do it, but if you have better solutions ... feel free to use yours instead.

[incyi]

Is it oke to name it sidebar.html, not sidebar.md ? I prefer .md , because other files are also .md in the project.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.