Point README at the published docs instead of restating them - #3

Summary

The Integrating custom properties with an external system guide is now live. It covers what external custom properties are, display names and namespacing, the permission model, and installation behavior — and it links to this repository as the example implementation.

That establishes the division this PR applies: the docs own the concepts and setup, this README owns configuring and running the server.

What changed

Five areas restated GitHub behavior that the published docs now own. The concern isn't length, it's that each was an independent statement of platform behavior that can drift as the feature changes.

That last one is the clearest example of the risk: the REST reference gained an authoritative statement of those rules the same day, so the README was a second, independently-maintained copy.

API Reference table

The table previously listed all six endpoints with links to their REST reference pages — the same six endpoints, with the same links, that the guide already lists under Create the automation.

Rather than delete it, the table now pairs each endpoint with the helper function in example-server/app.js that calls it, and the endpoint path itself carries the reference link:

This keeps the reference one click away while making the column that remains something only this repository can provide. It also makes the README's existing claim that "each endpoint maps to a helper function" navigable rather than asserted.

324 → 265 lines.

What deliberately did not change

All sample-specific content is untouched: Smee proxy setup, credential and .env configuration, npm install and the two-terminal run, expected terminal output, the helper function inventory, and webhook troubleshooting.

Prerequisites was also left alone. It shares a heading with the guide's Prerequisites, but the content doesn't overlap — the guide describes the people and roles involved, while this describes local tooling (Node 24, an organization, a webhook proxy).

Verification

• All 16 internal anchors resolve, and the table of contents matches the headings exactly
• All 11 distinct docs.github.com anchors referenced were confirmed present on the live pages
• Each endpoint-to-anchor pairing in the table was checked against x-github-internal.path and http-method in the operation definitions, not matched by path alone — two rows share a path and differ only by method (PATCH vs DELETE on …/values/{property_name})
• All six helper function names were confirmed to exist in app.js, and all six endpoint strings confirmed to be requested there
• npm run lint clean, 21/21 tests pass

One anchor broke during editing and was caught by the anchor check: Step 2 referenced the removed #write-only-apps-org-admin-controlled-namespacing subsection. It now points at the guide instead.