[WIP] add side nav

[sandrokeil]

@tobiju Please merge #15 first

Some open points here. Hope you can complete it.

• [ ] Scroll Spy Plugin doesn't work, but implemented
• [ ] Fixed side menu, so it's always visible
• [ ] Better mobile handling, maybe smaller?
• [ ] Maybe some style improvements

[tobiju]

Scroll Spy Plugin doesn't work, but implemented.

The get rid of this error you need to replace the dots in the anchors and links.