[Topic] Alt text for images

[rianrietveld]

URL: https://wpaccessibility.org/docs/topics/content/alt-text/
Code: https://github.com/wpaccessibility/wp-a11y-docs/blob/main/docs/topics/content/alt-text.md

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This content should be reviewed and expanded and maybe divided into separate topic pages.