Add deb822 apt sources to install section in Debian manual

[LLdaniel]

Description

With Debian Trixie released the new apt sources format RFC822 is now the default.
This PR proposes a way to take this new format into account.

The goal would be to have the follwoing docker.sources:

$ cat /etc/apt/sources.list.d/docker.sources 
Types: deb
URIs: https://download.docker.com/linux/debian/
Suites: trixie
Components: stable
Signed-By: /etc/apt/keyrings/docker.gpg

Related issues or tickets

#15382 and #23231

Reviews

I am happy to change my PR if needed. Currently I was not sure if we should keep both formats or make the new deb822 format as default.
Maybe we could move the legacy format into an info box?

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[eshattow]

deb822 format was introduced in apt version 1.1 ten years before the release of Trixie

[LLdaniel]

oh 😅 , thanks for that comment.
Did not know that, I can adjust the comment together with possible other review comments.

Maybe we should just say:

 # Add the repository to Apt sources (deb822 format)

[eshattow]

Drop this comment and the use of non-deb822 format sources

[LLdaniel]

Addressed with 7e2650e

(Hope it is still ok to keep the original comment, which just stated # Add the repository to Apt sources:. The paragraph seemed to me a bit lonely without the comment. At least explanations about (non-)deb822 are gone now).