fix typo

The installation documentation currently implies that all commands are
being prompted by the root user or an user with root-like privileges.
This is now explicitly stated and, additionally, another if-guard was
added to not include the "Add Icinga Package Repository" section for
Windows, as it does not fit there.

Closes #9959.

just like on packages.icinga.com and for the same security reasons we don't
use apt-key add anymore for: https://blog.cloudflare.com/dont-use-apt-key

Our repos already reference https://packages.icinga.com/icinga.key
and both RPM distro families properly handle that.

The Backports Repository is required for Debian 9, but we don't build even v10 anymore.

… Pi OS

We instruct the users to build as root and chown just /usr/local/icinga2/var, but at least `icinga2 api setup` also needs to modify /usr/local/icinga2/etc.

It's EOL.

feat: Add the `$` character to the escape sequences table.

This is a good alternative to `icinga2 feature enable debuglog`:

* Object creation/deletion via API happens immediately and requires no restart
* Hence, the debug log is enabled exactly as long as desired

Co-authored-by: alvar <8402811+oxzi@users.noreply.github.com>

No packages to be installed according to these instructions require the given repo.