docs/annoations: add policy example and a link to the policy reference (#5180)
docs/annoations: add policy example and a link to the policy reference (#5180)

Added Rego subheading to the annotations documentation section and
added an example of using annotations, then below that a link to the
Rego policy reference.

Fixes #4937.

Signed-off-by: Peter Macdonald <4545cf630c5fb4f68d5ab3984d01bd385e167283@gmail.com>
open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa