doc: use more modern fonts

Use Noto as baseline font and Inconsolata Nerd Font for source code.
These are both esthetically pleasing and have good Unicode coverage.

NOTE: Unsupported characters (i.e. emojis, at the time of this commit)
are rendered as tofu (i.e. a square).

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>