Add `mdbook` to flake.nix (#15545)

changelog.d/15532.misc

@@ -1 +1 @@
-Install the `xmlsec` package and switch back to the upstream [cachix/devenv](https://github.com/cachix/devenv) repo in the nix development environment.
+Install the `xmlsec` and `mdbook` packages and switch back to the upstream [cachix/devenv](https://github.com/cachix/devenv) repo in the nix development environment.

changelog.d/15533.misc

@@ -1 +1 @@
-Install the `xmlsec` package and switch back to the upstream [cachix/devenv](https://github.com/cachix/devenv) repo in the nix development environment.
+Install the `xmlsec` and `mdbook` packages and switch back to the upstream [cachix/devenv](https://github.com/cachix/devenv) repo in the nix development environment.

changelog.d/15545.misc

@@ -0,0 +1 @@
+ Install the `xmlsec` and `mdbook` packages and switch back to the upstream [cachix/devenv](https://github.com/cachix/devenv) repo in the nix development environment.

flake.nix

@@ -97,6 +97,9 @@
 
                   # Native dependencies for running Complement.
                   olm
+
+                  # For building the Synapse documentation website.
+                  mdbook
                 ];
 
                 # Install Python and manage a virtualenv with Poetry.