docs/website: add Othermo as a silver sponsor

Othermo offers manufacturer-independent operational monitoring for
energy centers and boiler rooms. They connect a wide range of peripheral
such as meters, pumps or pressure maintenance [1][2].

Thank you for sponsoring LTS maintenance !

[1] https://othermo.de/
[2] https://www.linkedin.com/company/othermo-gmbh/

Signed-off-by: Thomas Perale <thomas.perale@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/website/sponsors.html

@@ -68,6 +68,20 @@
             </div>
           </div>
         </div>
+        <div class="sponsor-entry">
+          <div class="panel panel-default panel-lts-sponsor">
+            <div class="panel-body">
+              <div class="sponsor-title">
+                <a href="https://othermo.de/">
+                  <img class="img-responsive center-block" style="width: 100%;" src="images/othermo-logo.png"/>
+                </a>
+              </div>
+              <div class="sponsor-body">
+                <a href="https://othermo.de">Othermo</a>
+              </div>
+            </div>
+          </div>
+        </div>
       </div>
     </div>