Upgrade dependencies, including pa11y-ci to 4 from 2

[danyalaytekin]

Main changes

• bump pa11y-ci to 4 from 2
• use Node 20 up from 8
• use Ruby 3 up from 2 and update some deps
• describe how to install Ruby and Node stuff and to validate/test workflows locally
• some tidying up
  • use Pa11y's .editorconfig
  • gently forbid asdf because it caused me some trouble a while back here
  • add command make browse (jekyll serve --open is available but adding --open it would interfere with build and it can't be run again without restarting server)
  • build badge

[jpw]

I know nothing about Ruby so unable to assist with reviewing this one sadly.

[hollsk]

Looks GREAT to me, thanks @danyalaytekin 🙏

[sonniesedge]

What idiot asked me to review something? 😂

[sonniesedge]

What idiot asked me to review something? 😂

Oh hai @danyalaytekin 😂

[danyalaytekin]

Thanks for your earlier 2023 approval @hollsk! pa11y-ci has moved on again, so I've updated its version and made a few other tweaks to allow the project to build and pass its tests. I'll merge this now but let me know if there's any additional feedback. P.S. The CC image on the homepage appears to have gone missing - I haven't fixed this here.