Replace AnchorJS with a Hugo render hook

TODO:

• move the levels to config.yml
• a11y
• maybe use ::after?
• add padding?
• fix :focus if possible
• test that there are no regressions

Idea taken from https://github.com/gohugoio/gohugoioTheme/pull/146

Pros:

• No JS
• ...

Cons:

• Works only with Markdown headings, but on the other hand our ToC only works with Markdown already
• ...

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Preview: https://deploy-preview-32953--twbs-bootstrap.netlify.app/docs/5.1/getting-started/introduction/