docs: point plugin setup links at the sections that replaced the removed guides

[tonydzi]

hi, i am an AI agent (Claude) opening this on behalf of Anton Dzyatkovsky, who reviews what i open.

Description

The "Documentation" list under the Claude Code plugin section of the root README links to plugins/claude-code/FIRST-TIME-SETUP.md and plugins/claude-code/QUICK-START.md. Both files were removed in #225 (2026-02-19, "overhaul Claude Code plugin for correctness and spec compliance"), so both links have been 404 on github.com since then. The root README is the first thing a new plugin user opens, and these are the two links that promise to get them started.

The plugin README absorbed that content: it has an Installation section (marketplace and local options) and a Quick Start section. This PR repoints the two links at those sections and keeps the list otherwise as it was.

Type of Change

• New feature (agent, command, tool)
• Bug fix
• Documentation
• Refactoring
• CI/CD

Checklist

• Tests pass locally (markdown only, no code touched)
• Documentation updated (if needed)
• Follows CONTRIBUTING.md

Testing

• curl on the blob URLs from main: plugins/claude-code/FIRST-TIME-SETUP.md 404, plugins/claude-code/QUICK-START.md 404, plugins/claude-code/README.md 200.
• Rendered anchor ids on the plugin README: user-content--installation and user-content--quick-start, which is what #-installation and #-quick-start resolve to (GitHub drops the emoji from the heading slug).
• git log on the two removed paths: last touch is fix: Claude Code plugin overhaul — spec compliance, bug fixes, skill improvements #225, status removed. No other file in the tree references either name.