Added the install-fortran.md file.  It will eventually be moved into the fortran project as well.
Initial split of Guide from rest of documentation.  Using 'tagfiles' to cross-reference from one documentation set to the other works, but then it drags all of the 'external' related pages into the index of the other.
Modified visability of the external groups and pages.
Adding proper section names to 'Example' sections, to avoid Doxygen warnings.
Merge branch 'sub_doc' of https://github.com/Unidata/netcdf-c into sub_doc