Link to new Java sync 4.3 docs site (#736)

Author: Chris Cho

docs/reference/themes/mongodb/layouts/partials/header.html

@@ -25,3 +25,6 @@
               <div class="documentwrapper">
                 <div class="bodywrapper">
                   <div class="body">
+                    {{ if ( findRE ".*mongo-java-driver\/4.3\/(?!apidocs)" .URL ) }}
+                      {{- partial "header/newSiteNotice.html" . -}}
+                    {{ end }}

docs/reference/themes/mongodb/layouts/partials/header/newSiteNotice.html

@@ -0,0 +1,6 @@
+<div style="padding: 15px; margin: 20px 0px; border-width: 1px 1px 1px 0px; border-style: solid; border-radius: 6px; font-size: 14px; line-height: 20px; color: rgb(17, 97, 73); border-color: rgb(195, 231, 202) rgb(195, 231, 202) rgb(195, 231, 202) rgb(19, 170, 82); background-color: rgb(228, 244, 228);">
+  This reference documentation site is for versions of the MongoDB Java synchronous driver up to 4.3-beta.
+  <br/>
+  See the <a style="font-weight: bold" href="https://docs.mongodb.com/drivers/java/sync/current">MongoDB Java Driver documentation site</a> for version 4.3 and later versions of the reference documentation.
+</div>
+