doc: extensions: Add Doxygen tooltips to main documentation

Add Doxygen tooltips in the main documentation.

The tooltips are triggered when hovering over C symbols in the
documentation and display a preview of the Doxygen documentation for the
symbol.

Some shadow DOM magic is needed to style the tooltips correctly, without
affecting the rest of the page.

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