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)

docs/guides/agent/agent_component_reference/parser.md

@@ -65,6 +65,12 @@ Starting from v0.22.0, RAGFlow includes MinerU (≥ 2.6.3) as an optional PDF p
    - If you decide to use a chunking method from the **Built-in** dropdown, ensure it supports PDF parsing, then select **MinerU** from the **PDF parser** dropdown.
    - If you use a custom ingestion pipeline instead, select **MinerU** in the **PDF parser** section of the **Parser** component.
 
+To use an external Docling Serve instance (instead of local in-process Docling), set:
+
+- `DOCLING_SERVER_URL`: The Docling Serve API endpoint (for example, `http://docling-host:5001`).
+
+When `DOCLING_SERVER_URL` is set, RAGFlow sends PDF content to Docling Serve (`/v1/convert/source`, with fallback to `/v1alpha/convert/source`) and ingests the returned markdown/text. If the variable is not set, RAGFlow keeps using local Docling (`USE_DOCLING=true` + installed package) behavior.
+
 :::note
 All MinerU environment variables are optional. When set, these values are used to auto-provision a MinerU OCR model for the tenant on first use. To avoid auto-provisioning, skip the environment variable settings and only configure MinerU from the **Model providers** page in the UI.
 :::