samples/llext/modules: build hello_world as built-in by default

This commit changes the default value of CONFIG_HELLO_WORLD_MODE to 'y'
so that the hello_world function is compiled as a built-in part of the
Zephyr image by default. This is the simplest possible configuration,
that works for all architectures.

To build hello_world as an llext module, the user must either follow the
commands in the documentation or set CONFIG_HELLO_WORLD_MODE=m in the
project file along with additional architecture-specific settings that
may be required for proper LLEXT support on the target.
Leave a note in the prj.conf and sample.yaml files to remind the user.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>