Add copyright notice to index footer of docs

Closes gh-742

docs/src/docs/asciidoc/index-docinfo-footer.html

@@ -0,0 +1,8 @@
+<div id="footer">
+    <div id="footer-text">
+        Version {spring-authorization-server-version}<br>
+        Last updated {docdatetime}<br>
+        Copyright &copy; 2020-{docyear}
+        <p class="paragraph">Copies of this document may be made for your own use and for distribution to others, provided that you do not charge any fee for such copies and further provided that each copy contains this Copyright Notice, whether distributed in print or electronically.</p>
+    </div>
+</div>

docs/src/docs/asciidoc/index-docinfo.xml

@@ -1,7 +1,7 @@
 <productname>Spring Authorization Server</productname>
 <releaseinfo>{spring-authorization-server-version}</releaseinfo>
 <copyright>
-<year>2020-2022</year>
+<year>2020-{docyear}</year>
 </copyright>
 <legalnotice>
     <para>

docs/src/docs/asciidoc/index.adoc

@@ -2,6 +2,8 @@
 = Spring Authorization Server Reference
 Joe Grandja, Steve Riesenberg
 v{spring-authorization-server-version}
+:docinfo: private-footer
+:nofooter:
 
 [horizontal]
 xref:overview.adoc[Overview] :: Introduction and list of supported features