feat(tvix/docs): add mdbook-admonish

This provides a plugin for callouts.

It needs to have additional CSS file added to `book.toml`,
which can be (re-)generated by `mdbook-admonish install`.

See https://github.com/tommilligan/mdbook-admonish/issues/171#issuecomment-2009166079
for more context.

Use it by adding a warning one to the architecture document.

Change-Id: I75c9a33d00acb603c6da10d3f9ce3485731c1672
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11805
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: yuka <yuka@yuka.dev>

tvix/docs/default.nix

@@ -10,6 +10,7 @@ pkgs.stdenv.mkDerivation {
 
   nativeBuildInputs = [
     pkgs.mdbook
+    pkgs.mdbook-admonish
     pkgs.mdbook-plantuml
     pkgs.plantuml
   ];