Rework REST API Documentation

[lucaderi]

While the REST API documentation is in relatively good shape now, it needs some little rework

• it is not clear what API REST calls are available according to the license. This could avoid headaches.
• it is not clear what API REST calls are historical or live. I suggest splitting the documentation into two sections
• some references are missing. For example, the various timeseries schemas used in timeseries are not listed
• for timeseries, it is not clear what schema parameters need to be passed
• it is not written anywhere whether POST or GET has to be used for calls

[MatteoBiscosi]

• POST/GET fixed
• references all fixed