minor symfony#16895 [RFC] Add alt texts to images (wouterj)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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[RFC] Add alt texts to images

I suddenly realized we didn't offer any alt texts for any of our images/diagrams. This PR adds 2 alt texts, to gather feedback on both if we want this and how we are going to provide alt texts for our diagrams (which can be quite complex to explain in words).

I used https://medium.com/nightingale/writing-alt-text-for-data-visualization-2a218ef43f81 and https://sc.edu/about/offices_and_divisions/digital-accessibility/guides_tutorials/alternative_text/charts-diagrams/index.php as two examples to help me write this text (this is my first time properly thinking about alt texts).

Commits
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673cd60 [RFC] Add alt texts to images

Author: wouterj