[Docs] Add documentation for the Role creation REST endpoint

In the CLI reference guide, we have the capability to create Roles [1]. However, this capability is missing in the REST guide  [2].

I've tried to use the method as defined in the code [3], and it works with this structure

$ curl -v -X PUT --digest -u "admin:password" localhost:11222/rest/v2/security/permissions/restrole?permission=ALL_READ&permission=ALL_WRITE

$ curl -X GET -u admin:password --digest localhost:11222/rest/v2/security/roles
["observer","application","clirole","admin","monitor","restrole","deployer"]

I understand that, if this is available through the CLI, this method is supported and in that case, its documentation should be trivial.

 [1]https://infinispan.org/docs/stable/titles/cli/cli.html#synopsis_43

 [2]https://infinispan.org/docs/stable/titles/rest/rest.html#rest_v2_security

 [3]https://github.com/infinispan/infinispan/blob/32c03896dcb0b936aad4caaa575e3185d032f503/server/rest/src/main/java/org/infinispan/rest/resources/SecurityResource.java#L97