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/partials/icons/bootstrap-stack.svg

@@ -0,0 +1 @@
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