docs: expand developer documentation and configuration guides

[Intina47]

Motivation

• The previous documentation was lightweight and lacked detailed, developer-focused guidance for setup, MCP configuration, Notion integration, and troubleshooting.
• Provide clearer onboarding and platform-specific examples so developers can install, configure, and debug Context Sync reliably.
• Surface important operational details (data paths, migration/backup behavior, git hooks, caveats) so the tool can be adopted by engineering teams with minimal friction.

Description

• Reworked README.md to add a concise “Why Context Sync?” section, improve install/auto-config notes, and index the richer docs.
• Dramatically expanded docs/CONFIG.md with a Manual Configuration Guide, platform-specific MCP snippets (Claude, Cursor, VS Code, Continue.dev, Zed, Windsurf, Codeium, TabNine, Codex CLI, Claude Code, Antigravity), validation checklist, and notes about npx vs global node usage.
• Expanded docs/TOOLS.md into a full developer tool reference describing the 9 core MCP tools, required tool flow, examples, remember caveat metadata, common errors, and MCP prompts/resources.
• Improved docs/DATA.md to describe local-first storage, default paths, custom DB options (--db-path and CONTEXT_SYNC_DB_PATH), migration/backup behavior, deletion, and security guidance.
• Updated docs/NOTION_INTERGRATION.md to clarify capabilities (read-only MCP support), step-by-step setup, sharing pages with the integration, and quick usage examples.
• Expanded docs/TROUBLESHOOTING.md with quick diagnostics, client-specific checks, clear core-tool errors, Notion troubleshooting, git-hook guidance, migration notes, and a ‘still stuck’ recovery step.
• Added a short release-note entry in docs/RELEASE_NOTES.md calling out the documentation expansion; committed all changes.

Testing

• No automated tests were added or run because these are documentation-only changes and the project has no automated docs test suite.

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