man: Add weston-bindings(7) describing desktop shell shortcuts

The desktop-shell-specific bindings were identified from
shell_add_bindings in desktop-shell/shell.c. Various general shortcuts
(like Ctrl+Alt+F) are provided by files in libweston/ and compositor/ .

Also introduce references to the new manual in weston(1) and weston.ini(5).

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>