User documentation REST API

To not be confused with documentation of all API calls (JavaDoc / Swagger). We need a page in EJBCA Documentation space describing and giving a overview of the new REST API, similar to the JAX-WS documentation page.

Items to include:

• Short intro describing usage
• Getting Started
• Available URL resources
• How to integrate with the API (i.e. how to request it from external clients)
• Authentication
• Workflow examples
• Error Codes