docs: add signer-node HTTPS proxy guidance

[unclekaldoteth]

Fixes #1627

Summary

• Add documentation for running signer and stacks-node over encrypted links when they are hosted on separate machines.
• Document a reverse-proxy pattern that preserves required IP:PORT config fields while adding TLS between hosts.
• Add an explicit operator validation checklist.

Changes

• docs/operate/run-a-signer/README.md
  • Added an Optional: TLS between signer and node on separate hosts section.
  • Added traffic-path mapping for both directions:
  • stacks-node -> signer events flow through local proxy + HTTPS.
  • stacks-signer -> stacks-node RPC flow through local proxy + HTTPS.
  • Added concrete signer-config.toml and node-config.toml examples using loopback proxy endpoints.
  • Added Nginx examples for node host and signer host.
  • Added validation checklist (curl, certificate verification, logs).
• docs/reference/node-operations/signer-configuration.md
  • Added guidance that node_host and events_observer.endpoint remain IP:PORT values (not https://...).
  • Added note to use local loopback proxies for TLS and linked to run guide.

Testing

npx --yes markdownlint-cli --disable MD001 MD013 MD022 MD024 MD031 MD034 MD040 -- docs/operate/run-a-signer/README.md 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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