feat: add comprehensive documentation including user guide, API reference, and deployment instructions

docs/API.md

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+# API Documentation
+
+DocuNova provides a powerful REST API for programmatic access to all its features. This document serves as a reference for the available endpoints and their usage.
+
+## API Overview
+
+- Base URL: `http://<your-docunova-instance>/api`
+- Authentication: OAuth2 (when enabled)
+- Response Format: JSON
+
+## Interactive API Documentation
+
+The most up-to-date and interactive API documentation is available at:
+
+`http://<your-docunova-instance>/docs`
+
+This Swagger UI provides a complete reference with the ability to try out API calls directly from your browser.
+
+## Authentication
+
+When authentication is enabled, you must include an authentication token in your requests:
+
+```bash
+curl -X GET "http://<your-docunova-instance>/api/files" \
+  -H "Authorization: Bearer <your-token>"
+```
+
+## Common Endpoints
+
+### Document Upload
+
+**POST** `/api/upload`
+
+Upload one or more files for processing.
+
+**Request**: 
+- Multipart form data with file(s)
+
+**Response**:
+```json
+{
+  "success": true,
+  "file_ids": [123, 124],
+  "message": "Files uploaded and queued for processing"
+}
+```
+
+### Get Files
+
+**GET** `/api/files`
+
+Retrieve a list of processed files.
+
+**Parameters**:
+- `limit` (optional): Maximum number of files to return
+- `offset` (optional): Pagination offset
+- `search` (optional): Search term
+
+**Response**:
+```json
+[
+  {
+    "id": 123,
+    "original_filename": "invoice.pdf",
+    "file_size": 1024000,
+    "mime_type": "application/pdf",
+    "created_at": "2023-04-15T12:30:45Z"
+  },
+  ...
+]
+```
+
+### File Metadata
+
+**GET** `/api/files/{file_id}/metadata`
+
+Retrieve metadata for a specific file.
+
+**Response**:
+```json
+{
+  "document_type": "invoice",
+  "date": "2023-04-10",
+  "vendor": "Acme Corp",
+  "amount": "$1,234.56",
+  "extracted_text": "..."
+}
+```
+
+### Process Control
+
+**POST** `/api/files/{file_id}/reprocess`
+
+Reprocess a specific file.
+
+**Response**:
+```json
+{
+  "success": true,
+  "message": "File queued for reprocessing"
+}
+```
+
+## Error Handling
+
+Errors follow standard HTTP status codes with descriptive messages:
+
+```json
+{
+  "detail": "File not found",
+  "status_code": 404
+}
+```
+
+## Rate Limiting
+
+The API implements rate limiting to ensure system stability. If you exceed the limits, you'll receive a `429 Too Many Requests` response.
+
+## Client Libraries
+
+For easy integration with your applications, consider using one of our client libraries:
+
+- Python: [github.com/christianlouis/docunova-python-client](https://github.com/christianlouis/docunova-python-client)
+
+## Further Assistance
+
+For additional help with the API, please contact our support team or refer to the [Development Guide](../CONTRIBUTING.md).