docs: prompts in shell sessions shouldn’t be selectable

these just indicate the start of the prompt by convention & if the user
is root or not with `#` & `$` respectively

Change-Id: Id65f5f879b067ee96715b3e7dc63f68c7ad791b8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11868
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/docs/book.toml

@@ -22,4 +22,4 @@ after = ["links"] # ensure `{{#include}}` also gets processed
 [output]
 
 [output.html]
-additional-css = ["./mdbook-admonish.css"]
+additional-css = ["./mdbook-admonish.css", "./mdbook-extra.css"]

tvix/docs/mdbook-extra.css

@@ -0,0 +1,7 @@
+@charset "utf-8";
+
+.hljs-meta.prompt_ {
+	-webkit-user-select: none;
+	-moz-user-select: none;
+	user-select: none;
+}