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/data/breakpoints.yml

@@ -0,0 +1,29 @@
+- breakpoint: xs
+  abbr: ""
+  name: Extra small
+  min-width: 0px
+  container: ""
+
+- breakpoint: sm
+  abbr: -sm
+  name: Small
+  min-width: 576px
+  container: 540px
+
+- breakpoint: md
+  abbr: -md
+  name: Medium
+  min-width: 768px
+  container: 720px
+
+- breakpoint: lg
+  abbr: -lg
+  name: Large
+  min-width: 992px
+  container: 960px
+
+- breakpoint: xl
+  abbr: -xl
+  name: Extra large
+  min-width: 1200px
+  container: 1140px