Short set of accessibility skills

https://github.com/addyosmani/web-quality-skills/blob/main/skills/accessibility/SKILL.md

Now this might be too long https://github.com/mgifford/accessibility-skills

But WCAG is complicated and accessibility is even more complicated.

Open `skills/accessibility/SKILL.md` first to inspect the current accessibility skill list and how it is presented. Compare it with the linked `mgifford/accessibility-skills` example to decide what to trim into a shorter format. Update the markdown in that file (and any referenced index if needed) so it stays a concise, usable accessibility skill set; no tests are mentioned in the issue.

索引モデルが issue の本文から書いたものです。