Accessibility Fixes

[pancakereport]

Accessibility fixes to address the results of the a11y GitHub Action. Main issues include

• Violation of "image-alt"
• Violation of "scrollable-region-focusable"
• Violation of "color-contrast"

Scrollable-region-focusable and color contrast should both be solved upstream with Jupyter Book.

Color contrast issues appear when a cell intentionally raises errors (example here).

[pancakereport]

Scrollable-region-focusable and color-contrast are addressed upstream in Jupyter Book. jupyter-book/myst-theme#849 and jupyter-book/myst-theme#855

Based on what I'm seeing for other textbooks there may be some remaining issues, but I want to merge this for now.

[pancakereport]

Okay, looks like scrollable-region-focusable is still a problem for equations (9.5, 15.2, 18.1). Upstream issue: jupyter-book/myst-theme#858

And there's an odd color contrast issue that I'm having trouble tracking down, but might be a result of the latest myst-theme release

Testing http://localhost:8080/index.html ... please wait, this may take a minute.

  Violation of "color-contrast" with 1 occurrences!
    Ensure the contrast between foreground and background colors meets WCAG 2 AA minimum contrast ratio thresholds. Correct invalid elements at:
     - pre
    For details, see: https://dequeuniversity.com/rules/axe/4.11/color-contrast

I also see this issue for other Jupyter book deployments, like https://github.com/DS-100/debugging-guide/actions/runs/24542104174/job/72187152511