Navigation refactor

- add navigation for Core and More in header and footer
- add dropdown menu on click for Core and More
- fix docs navigation
- make navigation header fixed in project page (summary, docs, guides)
- fix "placeholder" for docs versions where "intro.md" was not present
yet
- added Core docs versions back to 1.2.6
Merge pull request #84 from SergioCrisostomo/navigation-menus

Add buttons to header and footer