Add Python script to approximate filtered h-gain waveform

[qwriter]

This PR includes the following updates:

• Add Python script that approximates h-gain waveforms filtered by the Advantage and Advantage2 system's low-pass filters.
• For better visibility of setting time-dependent gain for qubit biases, move much of the content from the description of the h_gain_schedule solver parameter to a new section in the QPU annealing topic.
• Remove note stating that the h_gain_schedule solver parameter is experimental only.
• Add references to papers that set time-dependent gain for qubit biases.

Question: Does the footnote about the supported slope still apply? (see this comment.)

The HTML for the h_gain_schedule solver parameter is here.

The HTML for the new section is here.