doc: more RTD theme tweaks

Change logo and increase logo size, remove RTD links to github source,
add CSS for :kbd: role (keyboard key press), change logo link to project
home rather than doc home (via zephyr-custom.js). Update 404 error page
for wider page layout, force page break before footer, show URL that got
you to the 404 page.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>