doc: add examples section to earthdistance documentation

Hi,

I noticed that the earthdistance documentation has no usage
examples, which makes it harder for new users to get started
with the module. Other contrib modules like hstore already
include helpful examples.

I've attached a patch that adds an Examples section covering
the main functions and the point-based distance operator.
All examples were tested on PostgreSQL 18.3.

Regards,
Dapeng Wang