add a table of contents for docs (#211)

docs/index.md

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+# Documentation
+
+For plugin overview and getting started guide, see [README](../README.md).
+
+## Table of Contents
+
+- [Core Concepts](concepts/index.md)
+
+  - [Block Bindings](concepts/block-bindings.md)
+  - [Field Shortcodes](concepts/field-shortcodes.md)
+
+- [Extending](extending/index.md)
+
+  - [Block Patterns](extending/block-patterns.md)
+  - [Block Registration](extending/block-registration.md)
+  - [Data Source](extending/data-source.md)
+  - [Hooks](extending/hooks.md)
+  - [Query](extending/query.md)
+  - [Query Runner](extending/query-runner.md)
+
+- [Workflows](workflows/index.md)
+
+  - [Airtable Integration](workflows/airtable-with-code.md)
+  - [Google Sheets Integration](workflows/google-sheets-with-code.md)
+  - [ZIP Code Integration](workflows/zip-code-with-contract.md)
+
+- [Development](local-development.md)
+- [Troubleshooting](troubleshooting.md)
+- [Releasing](releasing.md)
+
+## Additional Documentation
+
+- [Contributing Guidelines](../CONTRIBUTING.md)
+- [Security Policy](../SECURITY.md)
+- [Code of Conduct](https://make.wordpress.org/handbook/community-code-of-conduct/)