Cite launchpad bug that says ubuntu's pkgs are old (#14517)

* Cite launchpad bug that says ubuntu's pkgs are old

* Add some cross-references while I'm here

* Changelog

changelog.d/14517.doc

@@ -0,0 +1 @@
+Add links to third party package repositories, and point to the bug which highlights Ubuntu's out-of-date packages.

docs/setup/installation.md

@@ -84,7 +84,9 @@ file when you upgrade the Debian package to a later version.
 
 ##### Downstream Debian packages
 
-Andrej Shadura maintains a `matrix-synapse` package in the Debian repositories.
+Andrej Shadura maintains a
+[`matrix-synapse`](https://packages.debian.org/sid/matrix-synapse) package in
+the Debian repositories.
 For `bookworm` and `sid`, it can be installed simply with:
 
 ```sh
@@ -100,23 +102,27 @@ for information on how to use backports.
 ##### Downstream Ubuntu packages
 
 We do not recommend using the packages in the default Ubuntu repository
-at this time, as they are old and suffer from known security vulnerabilities.
+at this time, as they are [old and suffer from known security vulnerabilities](
+    https://bugs.launchpad.net/ubuntu/+source/matrix-synapse/+bug/1848709
+).
 The latest version of Synapse can be installed from [our repository](#matrixorg-packages).
 
 #### Fedora
 
-Synapse is in the Fedora repositories as `matrix-synapse`:
+Synapse is in the Fedora repositories as
+[`matrix-synapse`](https://src.fedoraproject.org/rpms/matrix-synapse):
 
 ```sh
 sudo dnf install matrix-synapse
 ```
 
-Oleg Girko provides Fedora RPMs at
+Additionally, Oleg Girko provides Fedora RPMs at
 <https://obs.infoserver.lv/project/monitor/matrix-synapse>
 
 #### OpenSUSE
 
-Synapse is in the OpenSUSE repositories as `matrix-synapse`:
+Synapse is in the OpenSUSE repositories as
+[`matrix-synapse`](https://software.opensuse.org/package/matrix-synapse):
 
 ```sh
 sudo zypper install matrix-synapse
@@ -151,7 +157,8 @@ sudo pip install py-bcrypt
 
 #### Void Linux
 
-Synapse can be found in the void repositories as 'synapse':
+Synapse can be found in the void repositories as
+['synapse'](https://github.com/void-linux/void-packages/tree/master/srcpkgs/synapse):
 
 ```sh
 xbps-install -Su