Add alt text to vignette images for pkgdown accessibility

[Copilot]

Add alt text to all vignette images

• Add fig.alt parameter to shinytest2.Rmd (4 images)
  • record-simple-app.png
  • record-simple-app-2.png
  • diffviewer-1.png
  • record-name.png
• Add fig.alt parameter to in-depth.Rmd (2 images)
  • screenshot-exports-app.png
  • screenshot-recorder-random-seed.png
• Add fig.alt parameter to robust.Rmd (1 image)
  • plot-app.png
• Add fig.alt parameter to use-application-audit.Rmd (2 images)
  • shinytest2-loadtest.png
  • shinytest2-shinyloadtest-optimized.png
• Add fig.alt parameter to using-monkey-testing.Rmd (6 images)
  • gremlins-start.png
  • gremlins-inject.png
  • gremlins-attack.gif
  • gremlins-slider-handle.png
  • gremlins-logs.png
  • gremlins-attack-refined.gif
• Verified all 15 images have fig.alt parameters
• Added NEWS entry for PR Add alt text to vignette images for pkgdown accessibility #435
• CI will verify no warnings from pkgdown::build_site()

Original prompt

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This section details on the original issue you should resolve

<issue_title>docs: Missing alt-text in vignettes</issue_title>
<issue_description>Run pkgdown::build_site(). There should be no warnings pertaining to Missing alt-text in vignettes

Learn more in vignette(pkgdown::accessibility).</issue_description>

<agent_instructions>Add alt text to all vignette images.

You can learn about the accessibility requirements here: https://pkgdown.r-lib.org/articles/accessibility.html

When fixed, there should be no alt text messages produced while running pkgdown::build_site()</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes docs: Missing alt-text in vignettes #422

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[schloerke]

@copilot Add a news entry. Your PR number is 435

[Copilot]

@copilot Add a news entry. Your PR number is 435

Added NEWS entry in commit 86e89be.