We broaden the Sage developer's guide, so that it gives guidance to users/developers how they can contribute, not limited to just the Sage library.

• Move https://trac.sagemath.org/wiki/WikiStart#Surveyingthemathematicalsoftwarelandscape to front page of developer's guide
• Merge https://wiki.sagemath.org/CodeSharingWorkflow into developer manual #34554 Merge https://wiki.sagemath.org/CodeSharingWorkflow into developer manual
• Add short chapter on distribution packaging

Preview of the new material: https://cfce091fecee537040d612098a22852d9f083c36--sagemath-tobias.netlify.app/developer/index.html

CC: @dimpase @kwankyu

Component: documentation

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/move__mathematical_software_landscape__from_trac_wiki_to_manual @ cfce091

Issue created by migration from https://trac.sagemath.org/ticket/34526