Add alt text guidance to the style guide

[cwickham]

Adds an "Images and Alt Text" section to _style-guide.md. The section has the rules that the recent alt text PRs used (#2138, #2195, #2199, #2203, #2206, #2210, #2211, #2213, #2217). New images can then get correct alt text when someone adds them.

Also adds .claude/rules/images-alt-text.md, a short form of the section. Claude Code loads this file when it edits a .qmd file or a .md file in docs/.

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[cwickham]

@cderv assigned a review to get your eyes on this. I'm happy with the content, I'm interested in your view on whether this split between rules/ and _style-guide.md is right.

[cderv]

I think this is good split yes.

Currently, I am learning more and experimenting with less context rules and more skills. So I wonder if we need a /quarto-alt-text or mor generic /alt-text skills like https://github.com/posit-dev/skills/tree/main/alt-text

Or maybe /style-guide should be the skill.

Anyhow, I think this is ok. We could try evaluate if this is working using evals in session using this PR rule, and other using default rules.

Did you try comparing ?

We can see at usage too.

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Successfully created backport PR for prerelease:

• [Sync to prerelease] Add alt text guidance to the style guide #2230