Insertion of templates in documentation files

[eileen-kuehn]

In #44 there was the question how code that is repeatedly used in documentation files could be reused. See metronome example in about (https://github.com/MaineKuehn/usim/blob/dff719c209ba8d275cd1133a0c447a361fce4486/usim/about.py#L15), index.rst (https://github.com/MaineKuehn/usim/blob/master/docs/index.rst) and README.rst (https://github.com/MaineKuehn/usim/blob/master/README.rst).