String interpolation in Redactor
Redactor fields are not meant to process twig both for security reasons (as they can be changed by any user) and for usability (you cannot expect authors to remember / preserve twig tags).

In your specific example, the site url is probably not necessary (starting a URL with / will link to the current domain just fine) - so you would not actually need twig.

However I cannot think of one good reason to build a menu in a redactor field. I would either include it directly in the page templates (.twig files) or use a matrix field as a content builder.