Latency Policy missing description and parameter

[tech-Nico]

The Latency Policy documentation (https://docs.gravitee.io/apim/3.x/apim_policies_latency.html) is missing a description therefore it's hard to tell what the policy does and how it can be used. Moreover, the policy actually allows you to specify the latency duration and this (rather important) detail is missing from the documentation.

Suggestion for a description:
The Latency Policy provides the ability to add latency to either the request or the response. So for example, if you configure the policy on the request with a latency of 100ms, the GW will wait 100ms before routing the request to the backend service.
This policy is particularly useful in two scenarios:

• Testing: adding latency allows you to test client applications when APIs are slow to respond
• Monetization: a longer latency could be added to free plans to encourage clients to move to a better or paid-for plan.

[plsstephenson]

This was done in PR: gravitee-io/gravitee-policy-latency#15. PR needs merging, then issue can be closed.