Document deb822 sources for Ubuntu 24.04 and later

Closes #1530.

The dependency instructions tell you to uncomment `deb-src` lines in
/etc/apt/sources.list. On Ubuntu 24.04 that file is a four line stub whose
own comments say the sources have moved, and there is nothing to uncomment.
A contributor following the guide gets:

    E: You must put some 'deb-src' URIs in your sources.list

Verified on Ubuntu 24.04.4 LTS. `apt-get build-dep -s python3` fails exactly
as above with the file as shipped. Adding `deb-src` to the `Types` field in
/etc/apt/sources.list.d/ubuntu.sources and running `apt-get update` makes it
resolve. Ubuntu's own generated file documents this at line 21:

    ## Types: Append deb-src to enable the fetching of source package.

The 22.04 one-line instructions are kept rather than replaced, since releases
still using that format need them.

Author: opensource-joe

getting-started/setup-building.rst

@@ -790,13 +790,32 @@ some of CPython's modules (for example, ``zlib``).
       dependencies for the Python you're working on by using the ``apt`` command.
 
       First, make sure you have enabled the source packages in the sources list.
-      You can do this by adding the location of the source packages, including
-      URL, distribution name and component name, to ``/etc/apt/sources.list``.
-      Take Ubuntu 22.04 LTS (Jammy Jellyfish) for example::
+      Where those live depends on your release.
+
+      On **Ubuntu 24.04 and later**, and on other releases using the deb822
+      format, the sources are in ``/etc/apt/sources.list.d/ubuntu.sources``.
+      ``/etc/apt/sources.list`` is left as a stub with nothing to uncomment.
+      Add ``deb-src`` to the ``Types`` field of the entries you want sources
+      for::
+
+         $ sudo nano /etc/apt/sources.list.d/ubuntu.sources
+
+      changing::
+
+         Types: deb
+
+      to::
+
+         Types: deb deb-src
+
+      On **Ubuntu 22.04 and other releases using the one-line format**, add the
+      location of the source packages, including URL, distribution name and
+      component name, to ``/etc/apt/sources.list``. Taking Ubuntu 22.04 LTS
+      (Jammy Jellyfish) as the example::
 
          $ deb-src http://archive.ubuntu.com/ubuntu/ jammy main
 
-      Alternatively, uncomment lines with ``deb-src`` using an editor, for
+      Alternatively, uncomment the lines with ``deb-src`` using an editor, for
       example::
 
          $ sudo nano /etc/apt/sources.list