Add Sphinx documentation and tutorials

The LISANode Simulation Model document is now starting to diverge from the LISA Instrument model, and does not contain any technical reference or tutorial or Quickstart sections. This has become an urgent need if the tool is to be used by a larger audience.

Instead, we should have an automatically generated (and versioned) documentation using Sphinx.

It should contain:

• Getting started section
  • A Quickstart page, for installation and a typical usage (simple simulation) with simple parametrization (sampling and size, orbits, glitch, GWs). A note on laser beam modeling (offset and fluctuations, carrier and sideband). A note on the measurements (quantities and units) one gets, and how to write the results to a measurement file. How to simply plots the beatnotes.
  • The underlying model, taking mostly what's in the LISANode Simulation Model document (conventions, time frames, laser beam model, propagation, interferometers, etc). Maybe split these sections into various pages, and move to reference.
  • A list of available noises, and default parametrizations.
• Reference section
  • Structure of measurement file.
  • Reference for Instrument.
  • DSP: mainly just API reference for timeshift().