Fix links in documentation sideview

[timroes]

What

Fixes #11313

This fixes all form of broken links in the documentation sideview:

• External Links will work properly again
• "Internal" links (i.e. links to another page in our own documentation) will now bring you properly to our documentation page
• Fragment only links (i.e. pointing to another part within the same documentation) will now cause the sideview to scroll properly to that part

In addition I modified the create react app development server to serve our documentation files the same way we bundle them in Docker, so documentation will now also work properly when using npm start.

[jamakase]

LGTM. Haven't tested locally.