build/pkgs: Add some missing install-requires metadata, add some documentation to the developer's guide #31321
Description
Dmitry Marakasov (repology.org) points out that some metadata are missing in our build/pkgs/.
repology/repology-updater#1118 (comment)
We fix this so that the SageMath distribution can be listed as a repository on repology.org
CC: @slel @dimpase @kiwifb @seblabbe @jhpalmieri @tobiasdiez
Component: build
Author: Matthias Koeppe
Branch/Commit: f829357
Reviewer: Dima Pasechnik
Issue created by migration from https://trac.sagemath.org/ticket/31321