Add 404 page

[froggleston]

Fixes #221

[netlify]

✅ Deploy Preview for carpentries-beta-handbook-preview ready!

Name	Link
🔨 Latest commit	8f39485
🔍 Latest deploy log	https://app.netlify.com/sites/carpentries-beta-handbook-preview/deploys/673f1f224a24870008498883
😎 Deploy Preview	https://deploy-preview-259--carpentries-beta-handbook-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[froggleston]

Ah this breaks horrifically due to sphinx not actually supporting custom 404 pages out of the box: https://sphinx-notfound-page.readthedocs.io/en/latest/

I'll adapt this PR to use the extension above instead.

[froggleston]

Still not working with the extension as it automatically injects prefixes to URLs e.g. /en/latest, and you cannot fully turn these off, i.e. setting the prefix configuration to None or "" still adds a leading / character which breaks things.

[maneesha]

@froggleston It is set up on our current sphinx site. I'm just not sure how. See https://docs.carpentries.org/xyz

[froggleston]

@maneesha The previous site uses a much older version of the extension that supports no prefix. However, unhelpfully, the extension devs removed this in later versions instead opting for "/" or a full prefix. There is no longer an option for no prefix. I can try the old version and see how it works.

[froggleston]

Strangely this seems to work now with the original extension.