v4: switch to Hugo

So, apart from Hugo being multiple times faster as we already know, I also switched to using Hugo to handle the docs assets (CSS/JS) like we do on main. Which means, it will be equally simple to work on the v4-dev branch as main. 😃

TODO:

• fix getting started snippets (also search for site.)

• replace the remaining Liquid code

• fix jQuery being bundled in the docs JS; not sure how to handle this without hardcoding the files we want to include, since we are using Resources.match - EDIT: worked around it, not sure if there's a better solution

• make sure we use docsref when possible like in main

• Fix migration sidebar link

• Read browserslist, fonts, CSS variables etc

• Deduplicate npm scripts

• Fix duplicate trailing slashes in URLs; also run a full site link checker

• Fix ToC CSS - EDIT: added a patch but it seems there are more cases that we need to fix, with nested lists:

  Screenshot

  Before	After

  EDIT2: it was due to going from an h2 to h4; switched to h3 and now it's fine

• go through the layout code one more time; fix discrepancies like social logo, classes etc

• Figure out why some classes are missing: EDIT: this is right and previously was wrong

  • card-img is missing here https://deploy-preview-32077--twbs-bootstrap.netlify.app/docs/4.5/components/card/#horizontal this seems to affect main too
  • card-img-top becomes card-img here https://deploy-preview-32077--twbs-bootstrap.netlify.app/docs/4.5/components/card/#card-columns
  • class="mr-3" is missing here: https://deploy-preview-32077--twbs-bootstrap.netlify.app/docs/4.5/components/media-object/#nesting

• see if this bothers us a lot; I can't remove the newline there (although, we apply margin via CSS, so we could probably remove that if it bothers us a lot) https://deploy-preview-32077--twbs-bootstrap.netlify.app/docs/4.5/utilities/borders/#border-radius (it's the same on main too so we probably want to keep it)

  Screenshot

  Before	After

• see if we are going to go with the homepage patch or if I should drop it - EDIT: moved to #32196

• Update team members

• test all the stuff multiple times

Preview: https://deploy-preview-32077--twbs-bootstrap.netlify.app/