Add documents site (#1767)

* feat: add docs site

- Implemented dynamic routing for MDX documentation pages with language support.
- Created layout components for documentation with a header and footer.
- Added metadata for various documentation sections in English and Chinese.
- Developed initial content for the DeerFlow App and Harness documentation.
- Introduced i18n hooks and translations for English and Chinese languages.
- Enhanced header component to include navigation links for documentation and blog.
- Established a structure for tutorials and reference materials.
- Created a new translations file to manage locale-specific strings.

* feat: enhance documentation structure and content for application and harness sections

* feat: update .gitignore to include .playwright-mcp and remove obsolete Playwright YAML file

* fix(docs): correct punctuation and formatting in documentation files

* feat(docs): remove outdated index.mdx file from documentation

* fix(docs): update documentation links and improve Chinese description in index.mdx

* fix(docs): update title in Chinese for meta information in _meta.ts

frontend/src/content/en/reference/_meta.ts

@@ -0,0 +1,21 @@
+import type { MetaRecord } from "nextra";
+
+const meta: MetaRecord = {
+  "concepts-glossary": {
+    title: "Concepts Glossary",
+  },
+  "configuration-reference": {
+    title: "Configuration Reference",
+  },
+  "api-gateway-reference": {
+    title: "API / Gateway Reference",
+  },
+  "runtime-flags-and-modes": {
+    title: "Runtime Flags and Modes",
+  },
+  "source-map": {
+    title: "Source Map",
+  },
+};
+
+export default meta;

frontend/src/content/en/reference/api-gateway-reference.mdx

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+# API / Gateway Reference
+
+TBD

frontend/src/content/en/reference/concepts-glossary.mdx

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+# Concepts Glossary
+
+TBD

frontend/src/content/en/reference/configuration-reference.mdx

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+# Configuration Reference
+
+TBD

frontend/src/content/en/reference/runtime-flags-and-modes.mdx

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+# Runtime Flags and Modes
+
+TBD

frontend/src/content/en/reference/source-map.mdx

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+# Source Map
+
+TBD