Add alt text to all main documentation pages

[maxitg]

Changes

• Add alt text to all images in the main (not Yellowstone) documentation.
• Replace an image in CausalDensityDimension.md with TeX.

Comments

• This is needed to update markdownlint-cli to the latest version 0.44.0 without disabling rules.
• I tried to describe images in a way that the pages can be understood within seeing the images. In some cases that involved explicitly listing events in a token-event graph, which made the alt text pretty long. However, it's difficult to make them understandable otherwise. If you have a better idea, please let me know.

Examples

It is now possible to read documentation in links:

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This change is