feat(parser): support external Docling server via DOCLING_SERVER_URL (#13527)

### What problem does this PR solve?

This PR adds support for parsing PDFs through an external Docling
server, so RAGFlow can connect to remote `docling serve` deployments
instead of relying only on local in-process Docling.

It addresses the feature request in
[#13426](https://github.com/infiniflow/ragflow/issues/13426) and aligns
with the external-server usage pattern already used by MinerU.

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

### What is changed?

- Add external Docling server support in `DoclingParser`:
  - Use `DOCLING_SERVER_URL` to enable remote parsing mode.
- Try `POST /v1/convert/source` first, and fallback to
`/v1alpha/convert/source`.
- Keep existing local Docling behavior when `DOCLING_SERVER_URL` is not
set.
- Wire Docling env settings into parser invocation paths:
  - `rag/app/naive.py`
  - `rag/flow/parser/parser.py`
- Add Docling env hints in constants and update docs:
  - `docs/guides/dataset/select_pdf_parser.md`
  - `docs/guides/agent/agent_component_reference/parser.md`
  - `docs/faq.mdx`

### Why this approach?

This keeps the change focused on one issue and one capability (external
Docling connectivity), without introducing unrelated provider-model
plumbing.

### Validation

- Static checks:
  - `python -m py_compile` on changed Python files
  - `python -m ruff check` on changed Python files
- Functional checks:
  - Remote v1 endpoint path works
  - v1alpha fallback works
  - Local Docling path remains available when server URL is unset

### Related links

- Feature request: [Support external Docling server (issue
#13426)](https://github.com/infiniflow/ragflow/issues/13426)
- Compare view for this branch:
[main...feat/docling-server](https://github.com/infiniflow/ragflow/compare/main...spider-yamet:ragflow:feat/docling-server?expand=1)

##### Fixes [#13426](https://github.com/infiniflow/ragflow/issues/13426)

common/constants.py

@@ -219,6 +219,9 @@ class ForgettingPolicy(StrEnum):
 # ENV_MINERU_OUTPUT_DIR = "MINERU_OUTPUT_DIR"
 # ENV_MINERU_BACKEND = "MINERU_BACKEND"
 # ENV_MINERU_DELETE_OUTPUT = "MINERU_DELETE_OUTPUT"
+# ENV_DOCLING_SERVER_URL = "DOCLING_SERVER_URL"
+# ENV_DOCLING_OUTPUT_DIR = "DOCLING_OUTPUT_DIR"
+# ENV_DOCLING_DELETE_OUTPUT = "DOCLING_DELETE_OUTPUT"
 # ENV_TCADP_OUTPUT_DIR = "TCADP_OUTPUT_DIR"
 # ENV_LM_TIMEOUT_SECONDS = "LM_TIMEOUT_SECONDS"
 # ENV_LLM_MAX_RETRIES = "LLM_MAX_RETRIES"