docs: Add non-free-firmware component to backports repository

Adds the non-free-firmware component to the Debian backports
repository configuration. This component is required for
firmware packages on many systems.

The non-free-firmware component was split from non-free in
Debian 12 (Bookworm) to better support systems requiring
proprietary firmware while maintaining free software
principles for other non-free packages.

See: https://www.debian.org/releases/bookworm/amd64/release-notes/ch-whats-new.en.html#non-free-split
Signed-off-by: wommy <tom@technomad.media>

Author: wommy

docs/Getting Started/Debian/index.rst

@@ -23,8 +23,8 @@ Add the backports repository::
 
 .. code-block:: sourceslist
 
-   deb http://deb.debian.org/debian trixie-backports main contrib
-   deb-src http://deb.debian.org/debian trixie-backports main contrib
+   deb http://deb.debian.org/debian trixie-backports main contrib non-free-firmware
+   deb-src http://deb.debian.org/debian trixie-backports main contrib non-free-firmware
 
 ::