Refactor Debian/Ubuntu package

Move files for packaging outside the docs directory into its own
packaging directory. Replace the existing postinstall and postremove
scripts with Debian maintainerscripts to behave more like a typical
Debian package:

* Start and enable the headscale systemd service by default
* Does not print informational messages
* No longer stop and disable the service on updates

This package also performs migrations for all changes done in previous
package versions on upgrade:

* Set login shell to /usr/sbin/nologin
* Set home directory to /var/lib/headscale
* Migrate to system UID/GID

The package is lintian-clean with a few exceptions that are documented
as excludes and it passes puipars (both tested on Debian 12).

The following scenarious were tested on Ubuntu 22.04, Ubuntu 24.04,
Debian 11, Debian 12:

* Install
* Install same version again
* Install -> Remove -> Install
* Install -> Purge -> Install
* Purge
* Update from 0.22.0
* Update from 0.26.0

See: #2278
See: #2133
Fixes: #2311

mkdocs.yml

@@ -58,9 +58,6 @@ theme:
 
 # Excludes
 exclude_docs: |
-  /packaging/README.md
-  /packaging/postinstall.sh
-  /packaging/postremove.sh
   /requirements.txt
 
 # Plugins