Fix inconsistent axis labels in solimp documentation figures

[ManoharPaturi]

Body:

Fixes #3545

What does this PR do?

The solimp text in modeling.rst defines the impedance parameters as
$d_0$ and $d_{\text{width}}$ (since 83e7095), but the d(r) plot grid
(doc/images/modeling/impedance.png / impedance_dark.png) still labels
the vertical axis dmax / dmin — names that appear nowhere in the docs.

This regenerates both figures (light and dark) with the y-axis annotation
renamed to $d_0$ (bottom level, r = 0) and $d_{\text{width}}$ (top level,
r = ±width), matching the text. Everything else is reproduced faithfully:
same 3×3 panel grid (pow 1/2/6 × mid 0.1/0.5/0.9), same curve shapes
from the documented impedance interpolation, same dimensions (1580×1052),
same light/dark palettes, and the original labeling scheme (y labels only on
the top-left panel, x labels only on the bottom-left panel).

The curves use the impedance function as documented in the Modeling chapter:
d(r) = d₀ + (d_width − d₀) · S(|r|/width) with the power/midpoint spline
S(x) = 0.5·(x/y)^p for x ≤ y and 1 − 0.5·((1−x)/(1−y))^p otherwise.

How was it generated?

There is no figure-generation script in the repo, so the figures were
regenerated with the standalone matplotlib script below (happy to commit it
somewhere appropriate if you'd like to keep it):

generator script

import numpy as np
from PIL import Image
import matplotlib

matplotlib.use("Agg")
import matplotlib.pyplot as plt

WIDTH = 1.0


def d_of_r(r, d0, dwidth, midpoint, power):
    x = np.abs(r) / WIDTH
    s = np.where(
        x <= midpoint,
        0.5 * (x / midpoint) ** power,
        1.0 - 0.5 * ((1.0 - x) / (1.0 - midpoint)) ** power,
    )
    return d0 + (dwidth - d0) * s


def make_figure(theme):
    if theme == "light":
        bg, fg, grid_c, spine_c = "white", "black", "#efefef", "#929292"
    else:
        bg, fg, grid_c, spine_c = "black", "white", "#616161", "#dadada"

    plt.rcParams.update({
        "text.color": fg, "axes.labelcolor": fg,
        "xtick.color": fg, "ytick.color": fg, "font.size": 8.5,
    })

    fig, axes = plt.subplots(3, 3, figsize=(7.9, 5.26), dpi=200)
    r = np.linspace(-WIDTH, WIDTH, 1201)
    for row, mid in enumerate([0.1, 0.5, 0.9]):
        for col, power in enumerate([1, 2, 6]):
            ax = axes[row][col]
            ax.plot(r, d_of_r(r, 0.0, 1.0, mid, power), color=fg, linewidth=1.8)
            ax.set_title(f"pow {power}, mid {mid}", fontsize=8.5)
            ax.set_xlim(-1.25, 1.25)
            ax.set_ylim(-0.08, 1.08)
            ax.set_xticks([-1, 0, 1])
            ax.set_yticks([0, 1])
            ax.grid(True, color=grid_c, linewidth=0.7)
            for side in ("top", "right"):
                ax.spines[side].set_visible(False)
            for side in ("left", "bottom"):
                ax.spines[side].set_color(spine_c)
            ax.tick_params(length=3, labelsize=8)
            ax.set_xticklabels(
                ["-width", "0", "width"] if (row == 2 and col == 0) else ["", "", ""]
            )
            ax.set_yticklabels(
                [r"$d_0$", r"$d_\mathrm{width}$"] if (row == 0 and col == 0) else ["", ""]
            )
    fig.patch.set_facecolor(bg)
    fig.subplots_adjust(left=0.10, right=0.98, top=0.92, bottom=0.09, wspace=0.30, hspace=0.42)
    return fig


for theme, path in [
    ("light", "doc/images/modeling/impedance.png"),
    ("dark", "doc/images/modeling/impedance_dark.png"),
]:
    fig = make_figure(theme)
    fig.savefig(path, facecolor=fig.get_facecolor())
    plt.close(fig)
    if theme == "light":
        Image.open(path).convert("RGB").save(path)

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