Generate sitemap that can be used by browsers and indexers

[philipsens]

I tried to add Algolia search, but it is a bit tricky since it is not a static (pre-rendered) site. A sitemap may help.

[Matthbo]

This has been reverted as this step has to be done once the generated frankdoc json is available, meaning it has to be a step in the doclet instead of through the frontend build step