docs: Colorize callout and make the link work (#1445)

README.md

@@ -1,5 +1,12 @@
 # Santa
 
+> [!NOTE]
+> **As of 2025, Santa is no longer maintained by Google.**  We encourage
+> existing users to migrate to an actively maintained fork of Santa, such as
+> https://github.com/northpolesec/santa.
+
+---
+
 [![license](https://img.shields.io/github/license/google/santa)](https://github.com/google/santa/blob/main/LICENSE)
 [![CI](https://github.com/google/santa/actions/workflows/ci.yml/badge.svg)](https://github.com/google/santa/actions/workflows/ci.yml)
 [![latest release](https://img.shields.io/github/v/release/google/santa.svg)](https://github.com/google/santa/releases/latest)

docs/_config.yml

@@ -14,3 +14,7 @@ gh_edit_branch: "main"
 plugins:
     - jekyll-redirect-from
 
+callouts:
+  important:
+    title: Important
+    color: blue

docs/index.md

@@ -3,6 +3,11 @@ title: Home
 nav_order: 1
 ---
 
+{: .important }
+**As of 2025, Santa is no longer maintained by Google.**  We encourage existing
+users to migrate to an actively maintained fork of Santa, such as
+[https://github.com/northpolesec/santa](https://github.com/northpolesec/santa).
+
 # Welcome to the Santa documentation
 
 Santa is a binary and file access authorization system for macOS. It consists of a system extension that allows or denies attempted executions using a set of rules stored in a local database, a GUI agent that notifies the user in case of a block decision, a sync daemon responsible for syncing the database, and a server, and a command-line utility for managing the system.