doc: fix 404 page

Sphinx handling of 404 page is not that easy when using theme
customizations. Enabled the `sphinx-notfound-page` extension (maintained
by readthedocs) to fix its displaying.

Also adjusted Zephyr logo (was causing some Sphinx processing errors
when trying to scale it).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>