Add docs and changelog

Author: maxnoe

docs/api-reference/time.rst

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+.. _utils:
+
+**********************
+time (`~ctapipe.time`)
+**********************
+
+.. currentmodule:: ctapipe.time
+
+
+Reference/API
+=============
+
+.. automodapi:: ctapipe.time
+    :no-inheritance-diagram:

docs/changes/2707.datamodel.rst

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+Change format in which event timestamps are written to HDF5 files.
+Instead of a single float64 MJD value which has ~µs precision,
+the CTAO high precision time format is used now.
+This stores the timestamp as two uint32 values: seconds
+and quarter nanoseconds since ``1970-01-01T00:00:00.0 TAI``.
+
+This only affects the storage format and the precision,
+the in-memory API is unchanged as it relies on ``astropy.time.Time``
+and values are converted when reading/writing from/to HDF5.