Add firmware packaging guidelines to the documentation. The goal behind this was to make it so that it is very clear which packaging method is preferred for which cases to avoid fragmentation and inconsistency in pmaports packaging. Co-authored-by: Pablo Correa Gómez <pabloyoyoista@postmarketos.org> Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8161>