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)

rag/flow/parser/parser.py

@@ -32,6 +32,7 @@ from common import settings
 from common.constants import LLMType
 from common.misc_utils import get_uuid
 from deepdoc.parser import ExcelParser
+from deepdoc.parser.docling_parser import DoclingParser
 from deepdoc.parser.pdf_parser import PlainParser, RAGFlowPdfParser, VisionParser
 from deepdoc.parser.tcadp_parser import TCADPParser
 from rag.app.naive import Docx
@@ -173,7 +174,7 @@ class ParserParam(ProcessParamBase):
             pdf_parse_method = pdf_config.get("parse_method", "")
             self.check_empty(pdf_parse_method, "Parse method abnormal.")
 
-            if pdf_parse_method.lower() not in ["deepdoc", "plain_text", "mineru", "tcadp parser", "paddleocr"]:
+            if pdf_parse_method.lower() not in ["deepdoc", "plain_text", "mineru", "docling", "tcadp parser", "paddleocr"]:
                 self.check_empty(pdf_config.get("lang", ""), "PDF VLM language")
 
             pdf_output_format = pdf_config.get("output_format", "")
@@ -371,6 +372,29 @@ class Parser(ProcessBase):
                     "text": t,
                 }
                 bboxes.append(box)
+        elif parse_method.lower() == "docling":
+            pdf_parser = DoclingParser(docling_server_url=os.environ.get("DOCLING_SERVER_URL", ""))
+            lines, _ = pdf_parser.parse_pdf(
+                filepath=name,
+                binary=blob,
+                callback=self.callback,
+                parse_method=conf.get("docling_parse_method", "raw"),
+                docling_server_url=os.environ.get("DOCLING_SERVER_URL", ""),
+            )
+            bboxes = []
+            for item in lines:
+                if not isinstance(item, tuple) or not item:
+                    continue
+                text = item[0]
+                poss = item[-1] if len(item) >= 2 else ""
+                box = {
+                    "text": text,
+                    "image": pdf_parser.crop(poss, 1) if isinstance(poss, str) and poss else None,
+                    "positions": [[pos[0][-1], *pos[1:]] for pos in pdf_parser.extract_positions(poss)]
+                    if isinstance(poss, str) and poss
+                    else [],
+                }
+                bboxes.append(box)
         elif parse_method.lower() == "tcadp parser":
             # ADP is a document parsing tool using Tencent Cloud API
             table_result_type = conf.get("table_result_type", "1")