Add Mastodon verification linkback

Hidden/empty rel=me link to get our Docs Mastodon-verified

source/index.rst

@@ -1,3 +1,4 @@
+<a rel="me" href="https://fosstodon.org/@OpenDroneMap"></a>
 .. figure:: https://www.opendronemap.org/wp-content/uploads/2018/07/odm-logo.svg
    :alt: OpenDroneMap Logo
    :align: left
@@ -57,3 +58,4 @@ The documentation is available in several languages. Some translations are incom
     requesting-features
     contributing
     faq
+