doxygen: added removed feature document as page

doc/doxygen/riot.doxyfile

@@ -765,7 +765,8 @@ INPUT                  = ../../doc.txt \
                          src/creating-modules.md \
                          src/creating-an-application.md \
                          src/getting-started.md \
-                         src/changelog.md
+                         src/changelog.md \
+                         ../../LOSTANDFOUND.md
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses