Update documentation on utilities

[jlaehne]

Description of the change

• Added savetxt to utilities doc
• Moved crop_edges to signal-tools doc
• Added section mathematical routines in utilities doc

Progress of the PR

• [X] Change implemented (can be split into several points),
• [X] update user guide (if appropriate),
• [X] ready for review.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ca4d57d) 100.00% compared to head (8ebc91d) 100.00%.

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Additional details and impacted files

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