[Documentation] Clarify units and default values for analog parameters

[jpmoutinho]

Currently the usage of Analog blocks in Qadence requires some "experimental" knowledge of neutral atoms, e.g.:

• What spacing to use for the atoms (typically a user should set 8.0 micrometers by default).
• What duration the pulses should be (in nanoseconds).
• Variational parameters for the duration should be multiplied by 1000 because models converge better for parameters in microseconds, given that it's the unit of time used in all other constants.

These are mostly related to the unit system we use, which currently follows Pulser. Working with specific units can be very error-prone, and we should decide if we want to keep some units in Qadence or abstract them from the user as much as possible, and properly document it.

We could consider introducing a dependency for handling units like Pint or Magnitude. I wonder if any of them will work well if torch though?