Fix image rendering: add missing alt tags

[dawidl022]

The book at https://book.mlc.ai and the generated PDF were rendering the wrong images in places due to missing alt text, which is used by d2lbook for the figure captions.

This change adds alt text to all images which did not have it before, thus fixing the rendering issue.

I have made this change before reading the book, as such if there are some captions that I got wrong, I would appreciate corrections!

[gemini-code-assist]

Summary of Changes

Hello @dawidl022, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical documentation rendering issue where images in the online book and its generated PDF were displayed incorrectly due to missing alternative text. By systematically adding descriptive alt attributes to all affected images, the change ensures that the d2lbook system can correctly generate figure captions, thereby improving the overall accuracy and accessibility of the educational material.

Highlights

• Image Rendering Fix: Adds missing alt tags to image markdown syntax across several .md files to resolve rendering issues in the book and generated PDF. Previously, the absence of alt text, which d2lbook uses for figure captions, led to incorrect images being displayed.
• Documentation Clarity: Enhances the clarity and accessibility of the documentation by providing descriptive alternative text for all images, ensuring that the content is understandable even when images cannot be rendered or for users relying on screen readers.

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[gemini-code-assist]

Code Review

This is a great contribution that improves the accessibility and rendering of the book by adding the missing alt attributes to images. The alt texts are generally accurate and descriptive. I've made a couple of minor suggestions to make them even better and more consistent. I also noticed you've fixed a line wrapping issue, which is a nice touch. Thank you for this fix!

[dawidl022]

@gemini-code-assist comments addressed

[gemini-code-assist]

Thanks for addressing the comments, @dawidl022! The changes look good and significantly improve the documentation's accessibility and correctness. Great work!