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/layouts/shortcodes/param.html

@@ -0,0 +1,12 @@
+{{- /*
+  Work around wrong escapes in integrity attributes.
+*/ -}}
+
+{{- $name := .Get 0 -}}
+{{- with $name -}}
+{{- $value := $.Page.Param . -}}
+{{- if in $name "_hash" -}}
+  {{- $value = $value | safeHTML -}}
+{{- end -}}
+{{- with $value }}{{ . }}{{ else }}{{ errorf "Param %q not found: %s" $name $.Position }}{{ end -}}
+{{- else }}{{ errorf "Missing param key: %s" $.Position }}{{ end -}}