Fixed issues with sticky nav and mobile version

Description of changes:

#### Footer
* For smaller viewports the license tag may be cut off at the right, no content should go
beyond the right end of the document text. I have moved the license tag elsewhere.

#### Sticky Nav
* When the nav becomes sticky it jumps a little bit. This is caused by the `top: 10px` in
`.doc-nav.fixed`. To solve this issue and to keep the larger top padding for the nav box
a `padding-top: 30px` at `.doc-nav` should be placed.

* For smaller viewports the nav box may be partially obscured by the footer when scrolling
towards the end. Here the nav box needs an additional 3rd state, the states are then:
normal (top), fixed, normal (bottom). However, for larger viewports there should be no 3rd state because
the nav box may then jump towards the bottom of the site when scrolling down.

#### Mobile Version
* added padding to the text of the guides. (This was not possible for the other pages.)
* reduced the space above the footer
* added left padding to footer
* removed negative margin for "door"