Add code examples in timeseries module's docstrings

[datacubeR]

Adding code examples for:

• LagFeatures
• WindowFeatures
• ExpandingWindowFeatures

[codecov[bot]]

Codecov Report

Merging #647 (d871a2b) into main (c2f6152) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #647   +/-   ##
=======================================
  Coverage   97.91%   97.91%           
=======================================
  Files         100      100           
  Lines        3748     3748           
  Branches      726      726           
=======================================
  Hits         3670     3670           
  Misses         29       29           
  Partials       49       49           

Impacted Files	Coverage Δ
...imeseries/forecasting/expanding_window_features.py	89.18% <ø> (ø)
...ture_engine/timeseries/forecasting/lag_features.py	100.00% <ø> (ø)
...e_engine/timeseries/forecasting/window_features.py	100.00% <ø> (ø)

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