In spite of there being considerable documentation sometimes in the
config file, we go with CC0 because we don't want the notion of
'derived work' of a config file.
The example `settings.conf` is also CC0. Add some docs to
it while we're at it.
- Merge all the format documentation into netinstall.conf,
where the example is given in full as an embedded
*groups* entry.
- Get README.md to point to the example.
- Fix up headers in netinstall.yaml, pointing to the
embedded example in netinstall.conf.
- `local` is supposed to read from the config-file, rather than
externally; this simplifies examples, makes it easier to have
multiple netinstalls, and condenses the documentation.
- Document netinstall.conf a little,
- Add setting *required* which influences whether next is enabled or not
in case of missing or corrupt data,
- Enable *next* button only once some (any!) data is received.
This can be used to disallow stepping past the netinstall step when
there is no data (e.g. internet has failed between the welcome page
and the netinstall page).
