Add pluggy under new Developer Tooling section

[ch99q]

Summary

Adds pluggy — a standalone CLI for Minecraft plugin development (scaffolding, fetching server platforms like Spigot/Paper, running dev servers, building artifacts).

It didn't slot cleanly into any existing category:

• Not a library/framework — it's a CLI you run, not code you depend on.
• Not a plugin or mod — it doesn't run on a server or in the client.
• The closest existing bucket was Software → Tools, but those entries are end-user tools (Blockbench, MultiMC, PrismLauncher, pakkit), not developer tooling.

So this PR introduces a new Software → Developer Tooling subsection. It's a natural home for build tools, scaffolders, and other plugin/mod dev CLIs that don't fit elsewhere, and the section can grow over time. Happy to instead drop pluggy into Tools and skip the new subsection if you'd prefer — just let me know.

Changes

• New Developer Tooling entry in the table of contents under Software.
• New ## Developer Tooling subsection in the body, placed before Proxy Software.
• One entry: pluggy.