doc(mainpage): Adds mainpage for doxygen

This was discussed by the documentation group led by @Bai-Li-NOAA in
July (notes [here](
https://docs.google.com/document/d/1tPCSSanZ7SHaSE5SMkPoYrZh_lsswe7o5gtEyJmA0DU/edit?tab=t.0#heading=h.enmw68u5r5hm)).

inst/include/common/mainpage.dox

@@ -0,0 +1,13 @@
+/**
+ * @file mainpage.dox
+ * @mainpage C++ source code documentation
+ *
+ * This site contains C++ source code documentation for the Fisheries
+ * Integrated Modeling System (FIMS). Other forms of documentation are linked
+ * from the main documentation site at
+ * https://noaa-fims.github.io/FIMS/articles/fims-documentation.html.
+ *
+ * @copyright This file is part of the NOAA, National Marine Fisheries Service
+ * Fisheries Integrated Modeling System project. See LICENSE in the source
+ * folder for reuse information.
+ */