Writing guidelines & other docs should link to w3 "An alt Decision Tree" tutorial

[bsmth]

I can see we have one reference to this resource at web/html/element/img/index.md, but it would be great to link to this in the writing guidelines, too

• Page in question: https://developer.mozilla.org/en-US/docs/MDN/Writing_guidelines/Howto/Images_media#adding_alternative_text_to_images

• Link I expect to see: https://www.w3.org/WAI/tutorials/images/decision-tree/

Other places where this is appropriate in prose or See also:

• /learn/getting_started_with_the_web/html_basics/index.md
• /learn/html/multimedia_and_embedding/images_in_html/index.md
• /learn/accessibility/html/index.md
• /web/api/htmlinputelement/alt/index.md
• /learn/tools_and_testing/cross_browser_testing/accessibility/index.md

other resources:

• https://www.w3.org/WAI/tutorials/images/complex/