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.

[JoelPasvolsky]

Comment made before you switched to draft. Will review in full once it becomes a non-draft PR again.

[JoelPasvolsky]

This introductory sentence starts off with (non)limitations on the feature instead of what the feature is. Compare for example with the nice intro sentence in Pel2023:

We standardly start with an intro that gives the why of a feature rather than the how, and should do so here too.

[JoelPasvolsky]

There are two alternatives for feature descriptions in the Annealing Implementation and Controls that seem to me to follow good practices:

1. Single source these details
2. Refer users to h-gain-schedule parameter for the details; e.g., "For details on how to set h-gain for a problem, see the h-gain-schedule parameter"

Having two copies of these details, here and under h-gain-schedule parameter, risks discrepancies and adds maintenance costs.

Currently the Varying the Global Anneal Schedule section follows alternative 1, single sourcing the details with the anneal_schedule parameter. I actually think alternative 2 is better: the Annealing Implementation and Controls should explain what these features are used for and provide technical implementation details while the Solver Parameters section should specify usage rules. The former is for learning about the features and its hardware; the latter is a reference page for programmers.

[JoelPasvolsky]

The first sentence under this section (or a sentence before this section) needs to explain why we're talking about a filtered waveform:

The I/O system that delivers the h-controlling waveform to a QPU limits bandwidth using a low-pass filter; if you configure a too-rapidly changing curve, even within the supported bounds, expect distorted values of h for your problem

[JoelPasvolsky]

Consider the following structure for this section (rough draft). My concern with the current structure is that we have a very long section taken up with code to produce information unneeded by most researchers reading the Annealing Implementation and Controls, which is the critical physics page. To me it makes sense that for the occasional researcher who needs to get the filtered waveform, they can open a dropdown for the code.

QPU-Executed Schedule

A sentence here about how the input (nominal) schedule is filtered by the I/O and so the schedule executed on the QPU is bandwidth limited, as shown in the figure below.

You can use the provided script to approximate the schedule executed on the QPU.

.. dropdown:: Approximating the Filtered h-gain Waveform

    Here comes the script.

[JoelPasvolsky]

Forgot: we should make a PR to add this function to dwave-system, and once that is in the SDK, we would just refer to it here.