Archive v3 docs

[daun]

• When merging the next branch, the docs for v3 will only be accessible on a GitHub branch
• We should find a way of archiving and linking to the docs of v3
• Look into adding subdomains on JS.org — see example for v4.webpack.js.org
• Might have to fork the repo as a v3 archive going forward
• JS.org also accepts sites on Netlify

How important is this feature to you?

• [ ] Nice to have
• [x] Would make the lifes of current v3 users a lot easier
• [ ] I cannot use swup without it

[daun]

Done — v3.swup.js.org