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/book.toml

@@ -9,3 +9,13 @@ title = "Tvix Docs"
 # override the /usr/bin/plantuml default
 plantuml-cmd = "plantuml"
 use-data-uris = true
+
+[preprocessor.admonish]
+command = "mdbook-admonish"
+after = ["links"] # ensure `{{#include}}` also gets processed
+assets_version = "3.0.2" # do not edit: managed by `mdbook-admonish install`
+
+[output]
+
+[output.html]
+additional-css = ["./mdbook-admonish.css"]