Consolidate 16 wiki pages into Docs/ as the single source of truth, deduplicating content previously split across the wiki, CONTRIBUTING.md, and .claude/.

The motivation is to make docs easier to maintain with AI agents.
The key benefit of wiki is to more easily accept contributions and automatic menu navigation per page, but contributions have historically been zero and AI makes it much more efficient to manage docs.

The idea is to preserve wiki pages for a while, but link replace their content with links to these files instead.

New Docs/ structure:

• adding-a-new-unit.md, adding-operator-overloads.md, precision.md
• string-formatting.md, serialization.md, saving-to-database.md
• extending-with-custom-units.md, experimental-generic-math.md, nanoframework.md
• upgrading-from-{3,4,5}.x-to-{4,5,6}.x.md
• collaborators/{issues-and-pull-requests,releasing-nugets,regenerate-nuget-api-key}.md
• README.md (index)

Other changes:

• Fix backwards convention in conversion functions: prefer * for FromUnitToBaseFunc and / for FromBaseToUnitFunc (wiki had it reversed).
• Merge two overlapping NuGet release wiki pages into releasing-nugets.md.
• Skip Changelog wiki page (GitHub Releases is canonical) and Home (placeholder).
• CONTRIBUTING.md: slim down, link to Docs/ for detailed conversion/abbreviation rules.
• CLAUDE.md: add Documentation section linking to Docs/, reference adding-a-new-unit.md for conversion guidelines.
• .claude/criteria-for-adding-quantities-and-units.md: reference Docs/ instead of wiki.