add code examples to creation and datetime module's dosctrings

[datacubeR]

I added examples for the Creation Module and Datetime (since it is the only class on that module and functionality is similar).

[codecov]

Codecov Report

Merging #564 (321fe81) into main (b3ee503) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #564   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files          90       90           
  Lines        3492     3492           
  Branches      687      687           
=======================================
  Hits         3404     3404           
  Misses         32       32           
  Partials       56       56           

Impacted Files	Coverage Δ
feature_engine/creation/cyclical_features.py	100.00% <ø> (ø)
feature_engine/creation/math_features.py	97.77% <ø> (ø)
feature_engine/creation/relative_features.py	100.00% <ø> (ø)
feature_engine/datetime/datetime.py	100.00% <ø> (ø)

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[solegalli]

Great examples here @datacubeR, thank you so very much!!

I am merging as we speak!