Merge ~dbungert/curtin:deb822-doc into curtin:master

[Dan Bungert]

docs: link to sources.list manpage for apt deb822

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review: Approve (continuous-integration)

[Michael Hudson-Doyle (mwhudson)]

review: Approve

diff --git a/doc/topics/apt_source.rst b/doc/topics/apt_source.rst
index 299f5ca..0dfaa34 100644
--- a/doc/topics/apt_source.rst
+++ b/doc/topics/apt_source.rst
@@ -134,7 +134,8 @@ The file examples/apt-source.yaml holds various further examples that can be con
 deb822 sources on Ubuntu >= 23.10
 ---------------------------------
 
-By default, Ubuntu 23.10 and newer use the deb822 format for apt sources.
+By default, Ubuntu 23.10 and newer use the
+`deb822 format for apt sources <https://manpages.ubuntu.com/manpages/en/man5/sources.list.5.html>`_.
 When processing the apt configuration for a target system that should use deb822 sources, curtin will migrate legacy one-line sources to deb822 on-the-fly.
 The resulting configuration is functionally equivalent, but the sources on the target system will be formatted differently than provided in the configuration.