Add one more cross-link to help find the pkg.opam.template doc

[mbarbin]

Following the package-archiving-policy discussion this morning, I was looking for a way to add the field x-maintenance-intent to my opam files and landed on this page from a search:

https://dune.readthedocs.io/en/stable/reference/dune-project/generate_opam_files.html

I was later pointed to https://dune.readthedocs.io/en/stable/reference/packages.html#generating-opam-files which has more information.

I suspect that adding a little paragraph at the end of the first page saying More fields? (e.g. x-maintenance-intent, etc.) with a link to the section could help future users.

[Leonidas-from-XIV]

I agree this would be useful; can you submit a PR?