Switch to Hugo

This commit includes all the needed workarounds and most changes from the main branch for everything to work, like:

* removing empty lines in raw HTML that break output
* read .browserslistrc, CSS variables from disk instead of duplicating it
* using Hugo mounts
* using Hugo for the docs CSS/JS
* move ToC Sass code to a separate file while adapting it for Hugo

Thus, this patch makes our npm scripts faster since lint runs on one step and there's no separate docs assets processing.

site/assets/js/search.js

@@ -0,0 +1,60 @@
+// NOTICE!! DO NOT USE ANY OF THIS JAVASCRIPT
+// IT'S ALL JUST JUNK FOR OUR DOCS!
+// ++++++++++++++++++++++++++++++++++++++++++
+
+(function () {
+  'use strict'
+
+  var inputElement = document.getElementById('search-input')
+
+  if (!window.docsearch || !inputElement) {
+    return
+  }
+
+  var siteDocsVersion = inputElement.getAttribute('data-docs-version')
+
+  function getOrigin() {
+    var location = window.location
+    var origin = location.origin
+
+    if (!origin) {
+      var port = location.port ? ':' + location.port : ''
+
+      origin = location.protocol + '//' + location.hostname + port
+    }
+
+    return origin
+  }
+
+  window.docsearch({
+    apiKey: '5990ad008512000bba2cf951ccf0332f',
+    indexName: 'bootstrap',
+    inputSelector: '#search-input',
+    algoliaOptions: {
+      facetFilters: ['version:' + siteDocsVersion]
+    },
+    transformData: function (hits) {
+      return hits.map(function (hit) {
+        var currentUrl = getOrigin()
+        var liveUrl = 'https://getbootstrap.com/'
+
+        hit.url = currentUrl.lastIndexOf(liveUrl, 0) === 0 ?
+          // On production, return the result as is
+          hit.url :
+          // On development or Netlify, replace `hit.url` with a trailing slash,
+          // so that the result link is relative to the server root
+          hit.url.replace(liveUrl, '/')
+
+        // Prevent jumping to first header
+        if (hit.anchor === 'content') {
+          hit.url = hit.url.replace(/#content$/, '')
+          hit.anchor = null
+        }
+
+        return hit
+      })
+    },
+    // Set debug to `true` if you want to inspect the dropdown
+    debug: false
+  })
+})()