Some REST API documentation issues

[c8y3]

Here are a few issues with the REST API documentation.

• The example url provided in section "How to use" of https://docs.dfir-iris.org/operations/api/ has query parameter cid (http://localhost:8000/manage/cases/list?cid=1). But in the API documentation of endpoint /manage/cases/list, cid does not seem to be a query parameter.
• Field cid in the payload of /manage/cases/add is indicated as required. This does not seem to be the case both from the request sample and an experimentation with v2.0.0-beta-1.
• It would be nice to have a response sample for endpoint /case/export
• Endpoint /manage/customers/list to get the list of customers is not documented but seems to be working in version v2.0.0-beta-1
• Endpoint /manage/cases/{cid} to get a case seems not to be working in version v2.0.0-beta-1. It is not documented either, but it is present in the client. What should it be?

Initially opened as dfir-iris/iris-web#165 (comment)