Users frequently ask how to implement custom built-in functions and up
until now the information has been a bit hidden. This change improves
the visibility of the docs that describe how to customize/extend
OPA. We should revisit the section on how to customize the OPA daemon
to demonstrate how to use dep or modules as well as how to instantiate
only select portions of OPA (e.g., the plugin manager with a custom
gRPC server.)
Signed-off-by: Torin Sandall <torinsandall@gmail.com>