Stub configuration file refers to non-existant README documentation

[Charles Ferguson]

Steps to reproduce:

```
$ docker run -it --rm ubuntu:22.04
# apt-get update
...
# apt-get install -y cron-apt
...
(accept '5' local only configuration of postfix)
(supply system name as 'example.org')

root@6e12e552834c:/# cat /etc/cron-apt/config
# Configuration for cron-apt. For further information about the possible
# configuration settings see /usr/share/doc/cron-apt/README.gz.

root@6e12e552834c:/# cat /usr/share/doc/cron-apt/README.gz
cat: /usr/share/doc/cron-apt/README.gz: No such file or directory
root@6e12e552834c:/# ls /usr/share/doc/cron-apt/
changelog.gz copyright examples
```

Release of Ubuntu and package:

```
root@6e12e552834c:/# cat /etc/os-release
PRETTY_NAME="Ubuntu 22.04.1 LTS"
NAME="Ubuntu"
VERSION_ID="22.04"
VERSION="22.04.1 LTS (Jammy Jellyfish)"
VERSION_CODENAME=jammy
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=jammy
root@6e12e552834c:/# apt-cache policy cron-apt
cron-apt:
  Installed: 0.13.0+nmu1
  Candidate: 0.13.0+nmu1
  Version table:
 *** 0.13.0+nmu1 500
        500 http://archive.ubuntu.com/ubuntu jammy/universe amd64 Packages
        100 /var/lib/dpkg/status
```

What I expected ?

The configuration file refers to a file that doesn't exist. I'd expect the file to exist, or it to have a set of template configurations that I can fill in. Or to refer to other documentation that explained how to use it.

What happened instead ?

The file wasn't there. Leaving me a little unsure how to actually configure it.
I thought that maybe there's a '-docs' package to install to get the documentation, but none appears in the search:

```
root@6e12e552834c:/# apt-cache search cron-apt
cron-apt - automatic update of packages using apt-get
```

[Charles Ferguson (cferguson-undo)]

Aside:

Using `docker run -it --rm debian` instead (version 12.1), the README.gz is present in the doc directory.