Make convenience functionality more discoverable

[shumpohl]

The pulse building related current convenience functionality includes

• with_* methods like with_mapping
• pad_to method
• duration
• final_values and initial_values
• integral
• Arithmetics
• Time reversal

Currently these are either in their own example like https://github.com/qutech/qupulse/blob/master/doc/source/examples/00ArithmeticWithPulseTemplates.ipynb or are used in some examples when the author at the time thought of them.

• Add a paragraph or section in the documentation that
  • explains usecases for all of these methods by referring to
  • an FAQ like section where the usecases are demonstrated
• Add an example that showcases all convenience functionality.