ui: rename global 'Help' button to 'Documentation'

ui: rename global 'Help' button to 'Documentation'

To make it more clear that this will always end you on the
Documentation index.

Also, multiple Help buttons which all may or may not do something
different are a bit weird/confusing - thus distinction can here be
really helpful, IMO.

We target HD ready (1280x720) as minimal target resolution where this
still fits nicely. This (or bigger) resolution is also used by our
screenshot framework for the documentation - so it also fits there.

Adapt the icon displayed to `fa-book` which resembles the one often
used for manuals and is clearer than a '?', IMO.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Acked-by: Dominik Csapak <d.csapak@proxmox.com>