website: update sposoring state and include github sponsors link

Author: merge

website/index.html

@@ -75,13 +75,12 @@
 <a href="https://github.com/libts/tslib/blob/master/AUTHORS">actively
 maintained</a>.
 
-<center>
-<p style="font-size:11px">This site is operated with support from
-</p>
-<a href="https://www.ginzinger.com/en/" />
-<img src="ginzinger.svg" alt="Ginzinger electronic systems" style="width:29%; max-width:200px; padding-top: 8px;">
-</a>
-</center>
+<p>
+<b>Help keep tslib maintained</b>
+<p>
+tslib is largely developed and maintained by Martin in his spare time.
+If you use tslib, please help keeping it maintained by
+<a href="https://github.com/sponsors/merge">considering to donate</a>.
 </div>
 </body>
 </html>