[vitormattos]

Summary

• Update two n8n guide links to use the actively maintained shared example at LibreSign/documentation:main/examples/n8n/README.md, instead of a fixed commit.
• Keep this PR targeted to stable13 documentation; the source README and its relative 21-node workflow JSON remain together on documentation main.
• Remove the incorrect description that the README/JSON link is immutable.

Scope

• developer_manual/api/external-email-signers.rst
• developer_manual/integration/index.rst
• No copies of the shared example on stable documentation branches, and no changes to n8n JSON, tests, APIs, webhook catalog, or application code.

Version boundaries

• LibreSign 13 / application stable33 / documentation stable13; LibreSign 14 / application stable34 / documentation stable14; LibreSign 15 / application stable35 / documentation stable15.
• The shared example is deliberately maintained on documentation main, and selects payloads by installed LibreSign version. Release-specific cURL/PHP fixtures stay versioned in the respective documentation branches.
• This makes the shared link mutable. Compatibility with supported old versions still needs regression validation for future changes to the example.

Verification

• The original and cleaned example versions have byte-identical workflow JSON and n8n tests (previously verified Git blob SHAs: 16d0db680104d7cd219a847a1a9514f846043e1e, fbfa800d6ebb696cd107e4cea5c2ad05f28289bc).
• Earlier checks on the SHA-only PR revisions passed Sphinx with warnings as errors on four branches, 12 n8n tests, 29 JS tests, and 44 Python tests. These prior results do not prove this latest edit's CI.
• Verify current CI and HTML output before merging. No automatic merge.

Follow-up to #165–#168.