Actually add Doxygen main page

I apparently forgot to add this file in my previous commit (bd40ee2b).

doc/main_page.dox

+/** @mainpage The Minetest engine internal documentation
+
+Welcome to the Minetest engine Doxygen documentation site!\n
+This page documents the internal structure of the Minetest engine's C++ code.\n
+Use the tree view at the left to navigate.
+
+*/
+