docs: add guide for testing local CLI changes outside the monorepo (#137)

## Summary

Adds `docs/guides/testing-local-changes.mdx` — a contributor guide explaining how to test unreleased CLI changes against real projects outside the monorepo.

**Covers:**

- `pnpm link --global` (recommended — makes `hyperframes` in `$PATH` point at your local build)
- `node` alias (no PATH changes)
- `npm pack` (test the exact artifact that would be published)
- Troubleshooting (`which hyperframes`, port conflicts, stale builds)
- Table of test scenarios for each bug category

Also registers the page in `docs/docs.json` so it appears in the Guides nav.

docs/docs.json

@@ -82,8 +82,8 @@
             "pages": ["reference/html-schema"]
           },
           {
-            "group": "Community",
-            "pages": ["contributing"]
+            "group": "Contributing",
+            "pages": ["contributing", "contributing/testing-local-changes"]
           }
         ]
       }