fix: correct index.html indentation & draw piano image before the rest

- adapt some menu-item
- create href links for some menu-item
- fix indentation issues
- draw the piano image before the rest of the content by
  setting section positions to relative and removing image z-index
- use overflow to force reflow of #informations
- set minimum height to avoid the image to be truncated
feat: create a column system
fix: correct chrome issue

- each .inline menu were on the same stacking context
  unless a .toggle had been hovered
feat: add some opacity to the menu
feat: create inscription form

- add a palette variation
- create form style
- define layout h* font-weights
feat: create login form
feat: create contact form