Don't rely on relative path for docs preprocessor (#16883)

Reapplies #16700 with a corrected command. Now it no longer relies on a
relative path.

Thanks @maxdeviant for the quick help 🙏 

Release Notes:

- N/A

docs/book.toml

@@ -22,3 +22,12 @@ enable = false
 "/python.html" = "/docs/languages/python.html"
 "/adding-new-languages.html" = "/docs/extensions/languages.html"
 "/language-model-integration.html" = "/docs/assistant/assistant.html"
+"/assistant.html" = "/docs/assistant/assistant.html"
+
+# Our custom preprocessor for expanding commands like `{#kb action::ActionName}`,
+# and other docs-related functions.
+#
+# Comment the below section out if you need to bypass the preprocessor for some reason.
+[preprocessor.zed_docs_preprocessor]
+command = "cargo run -p docs_preprocessor --"
+renderer = ["html"]