Re-structure gammapy.time documentation

gammapy.time documentation must be deeply re-structured.

• the LightCurve object must be briefly presented with a reference to (gadf)[https://gamma-astro-data-formats.readthedocs.io/en/latest/lightcurves/index.html].
• a long presentation on periodicity using Lomb-Scargle periodograms should probably be removed for now. It should rather be included as a notebook.
• the LightCurveEstimator must be presented and its usage described briefly with a reference to the lightcurve.ipynb notebook.

[adonath]

After the generalisation of the FluxPoints class, I think the most useful thing to do here is add FluxPoints.to_table(format="binned-time-series"), such that the following works:

from gammapy.estimators import FluxPoints
from astropy.timeseries import BinnedTimeSeries

fp = FluxPoints.read()
data = fp.to_table(sed_type="flux", format="binned-time-series")
series = BinnedTimeSeries(data=data)

[adonath]

By now the gammapy.time sub-package was removed. So the remaining documentation is part of the gammapy.estimators (see https://docs.gammapy.org/0.17/estimators/lightcurve.html).

What could be useful is to show how users can convert the light curve into a BinnedTimeSeries object.