Fix outdated URLs

doc/footer.html

@@ -1,8 +1,8 @@
         </div>
         <div id="footer-container">
             <div id="footer">
-                Copyright © Laurent Gomila &nbsp;::&nbsp;
-                Documentation generated by <a href="http://www.doxygen.org/" title="doxygen website">doxygen</a> &nbsp;::&nbsp;
+                Copyright &copy; Laurent Gomila &nbsp;::&nbsp;
+                Documentation generated by <a href="https://www.doxygen.nl/" title="doxygen website">doxygen</a> &nbsp;::&nbsp;
             </div>
         </div>
     </body>

doc/mainpage.hpp

@@ -5,7 +5,7 @@
 /// Welcome to the official SFML documentation for C. Here you will find a detailed
 /// view of all the SFML <a href="./globals_func.htm">functions</a>.<br/>
 /// If you are looking for tutorials, you can visit the official website
-/// at <a href="http://www.sfml-dev.org/">www.sfml-dev.org</a>.
+/// at <a href="https://www.sfml-dev.org/">www.sfml-dev.org</a>.
 ///
 /// \section example Short example
 /// Here is a short example, to show you how simple it is to use SFML in C :