Installation instructions: Use deb822 format for additional APT sources

The current Mastodon installation instructions tell admins to download signing keys and add package source definition files to their system's APT config for PostgreSQL and Node.js.

However, the Debian trixie installation instructions suggest to already switch to a single debian.sources file in deb822 format (although the old format seems to be supported still ATM) and link to the sources.list manpage for more info.

My suggestion is to change the Mastodon installation instructions accordingly.

Side note: I don't think the Mastodon docs ever explained why PostgreSQL and Node.js must be sourced from non-Debian repos in the first place. At the time of writing, Mastdon v.4.5.7 requires PostgreSQL >= 13 and Node.js >= 20, trixie has PostgreSQL 17 and Node.js 20.19, so one should meet Mastodon's installation requirements with what is in the official Debian repos. Maybe the situation was different in former times.

If it is preferable to from the point of view of Mastodon devs to use more up-to-date versions (BTW I think it's a good idea, even if it complicates OS updates slightly), their reasoning should be documented accordingly.

Start with content/en/admin/install.md around lines 28-40 and compare its APT source instructions with the Debian trixie release notes and sources.list manpage linked in the issue. Update the installation guidance to reflect the requested source-file format and clarify the rationale for the PostgreSQL and Node.js repositories, then verify the rendered documentation and links.