Enhance README with detailed service description, setup instructions, and example .env configuration for the FastAPI service that integrates with Paperless-ngx and llama.cpp for PDF processing.

.env.example

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+PAPERLESS_BASE_URL="https://paperless.example.com"
+PAPERLESS_TOKEN="paste-token-here"
+
+LLAMA_BASE_URL="http://127.0.0.1:9292"
+LLAMA_MODEL="ggml-model-q4_k_m"
+
+PAPERLESS_CUSTOM_FIELD_NOTEBOOK_ID=1
+PAPERLESS_CUSTOM_FIELD_NOTEBOOK_PAGE=2
+
+# Document type id for each uploaded per-page document
+PAPERLESS_DOCUMENT_TYPE_ID=3
+
+PAPERLESS_TASK_TIMEOUT_S=600
+PAPERLESS_TASK_POLL_INTERVAL_S=5.0
+
+RENDER_DPI=200
+OCR_MAX_TOKENS=1024
+OCR_TEMPERATURE=0.0
+