doc: Reviews partitioning instructions to use parted.

The tests in <nixos/tests/installer.nix> are using `parted`, so they are
bound to be better tested than `fdisk`.

This is brought on by a couple issues, plus reports on IRC that the
`fdisk` instructions didn't work as expected.

 * #39354
 * #46309
 * #39942
 * #45478

Care was taken so that the other documented steps did not need changes.

In all this kerfufle, a slight re-organization of the Chapter has been
made, allowing better deep linking.