also updates nixdoc to 2.3.0. the nixdoc update is not a separate commit
because that would leave the manual build broken for one commit,
potentially breaking bisects and rebases.
This section was broken out into its own file in efb55d2a42 but
apparently never actually got included anywhere.
Since then a more detailed section on using unfree packages has been
introduced.
Copied the introduction and removed the rest of the file.
