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.

build/generate-sri.js

@@ -21,11 +21,11 @@ const pkg = require('../package.json')
 
 sh.config.fatal = true
 
-const configFile = path.join(__dirname, '../_config.yml')
+const configFile = path.join(__dirname, '../config.yml')
 
 // Array of objects which holds the files to generate SRI hashes for.
 // `file` is the path from the root folder
-// `configPropertyName` is the _config.yml variable's name of the file
+// `configPropertyName` is the config.yml variable's name of the file
 const files = [
   {
     file: 'dist/css/bootstrap.min.css',
@@ -40,7 +40,7 @@ const files = [
     configPropertyName: 'js_bundle_hash'
   },
   {
-    file: `site/docs/${pkg.version_short}/assets/js/vendor/jquery.slim.min.js`,
+    file: `site/static/docs/${pkg.version_short}/assets/js/vendor/jquery.slim.min.js`,
     configPropertyName: 'jquery_hash'
   },
   {