manager: some parts of documentation for version 6

See merge request !1377

Vladimír Čunát authored 2 years ago and Aleš Mrázek committed 1 year ago

The current code is really simple, so a whole summary fits easily.

- startup: link to Usage without the manager
- config: json-schema mention moved to config-overview

- legacy approach moved to Use Cases chapter
- legacy approach renamed to Usage without the manager

- installation: link to build from source
- startup: link to run without systemd

Added example of network interfaces configuration.

manager: python and poetry upgrade

See merge request !1400

I can't see why not.  Also avoid unnecessary /usr/bin/env
kresd doesn't work well without `ninja install` anyway.

bindir: maybe we should install the python binaries into sbin instead.
We're also being slightly inconsistent - allowing to override some
directories by meson but not respecting it for the python stuff.

6.0.x versions are dedicated to v6 alpha cycle.

Also sync NEWS with latest release 5.6.0 for consistency.