docs: fix broken signer configuration links

[unclekaldoteth]

Addresses #1729

Summary

• Fix broken internal links in signer configuration reference docs.
• Replace stale /broken/pages/... links with stable docs paths.
• Improve navigation from reference config examples to current operator guides.

Changes

• docs/reference/node-operations/signer-configuration.md
  • Replaced broken How to Run a Signer links with ../../operate/run-a-signer/.
  • Replaced broken Stacks Node Configuration link with ../README.md.

Testing

npx --yes markdownlint-cli --disable MD001 MD013 MD022 MD024 MD031 MD034 MD036 MD040 -- docs/reference/node-operations/signer-configuration.md
echo "exit_code=$?"
# exit_code=0

Checklist

• Required documentation updates included
• Changes are scoped to docs only
• Test evidence included

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