Package index request template

[awvwgk]

The packaging guidelines state:

https://github.com/fortran-lang/fortran-lang.org/blob/0013186256ed68901b883984731ee1e66e489642/PACKAGES.md#L55

But there is no PR template available for this purpose.

Additional context
Discussion regarding this line in fortran-lang/fortran-lang.org#26 (comment)