Add deprecation notice to the docs

Author: sandhose

book.toml

@@ -20,3 +20,5 @@ git-repository-url = "https://github.com/matrix-org/matrix-authentication-servic
 
 # The path that the docs are hosted on
 site-url = "/matrix-authentication-service/"
+
+additional-css = ["theme/additional.css"]

theme/additional.css

@@ -0,0 +1,20 @@
+#page-wrapper .page {
+    display: flex;
+    flex-direction: column;
+}
+
+.deprecation-notice {
+    box-sizing: border-box;
+    order: 2;
+    color: rgb(213, 25, 40);
+    background-color: rgb(255, 247, 246);
+    border: 1px solid rgb(213, 25, 40);
+    padding: 1rem 2rem;
+    margin: 1rem 0;
+    align-self: center;
+    max-width: var(--content-max-width);
+}
+
+#content {
+    order: 3;
+}

theme/header.hbs

@@ -0,0 +1,14 @@
+<div class="deprecation-notice content">
+    <h1>This documentation is out of date!</h1>
+    <p>
+        This documentation site is for the versions of matrix-authentication-service maintained by the <em>Matrix.org Foundation</em> (<a href="https://github.com/matrix-org/matrix-authentication-service">github.com/matrix-org/matrix-authentication-service</a>), available under the Apache 2.0 licence.
+    </p>
+
+    <p>
+        Since version 0.12.0, matrix-authentication-service is now maintained by <em>Element</em> under a new licence (<a href="https://github.com/element-hq/matrix-authentication-service">github.com/element-hq/matrix-authentication-service</a>).
+    </p>
+
+    <p>
+        If you are interested in the documentation for a later version of matrix-authentication-service, please refer to <a href="https://element-hq.github.io/matrix-authentication-service/">https://element-hq.github.io/matrix-authentication-service/</a>.
+    </p>
+</div>