Add toggle for "read mode" in UI

[Tschakki]

Is your feature request related to a problem? Please describe. Top bar is taking quite a lot of space, and in reality it will be barely used by the user. There should be the possibility to collapse the top nav to have more space for actual content on the screen.

Describe the solution you'd like There should be a toggle in the docs UI to switch between "normal" mode and "read" mode.

• In read-mode, the top nav will be collapsed, allowing the reader to use nearly the full height of the window for reading.
• Should be located in the toolbar, next to the "Edit" link.
• Should only be available for wide screens (only makes sense there)
• In read-mode, additional keyboard shortcuts are enabled, e.g. arrow keys for pagination