docs: heartbeats, Bun version, troubleshooting, and preview-branch cleanup (#3026)

Doc updates: 
- new Heartbeats page (yield, progress, external updates)
- Bun supported-version note
-  resource_exhausted troubleshooting with native builder link
- GitHub Actions preview-branch example with closed trigger so branches
archive when PRs close
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docs/docs.json

@@ -70,6 +70,7 @@
               "machines",
               "idempotency",
               "runs/max-duration",
+              "runs/heartbeats",
               "tags",
               "runs/metadata",
               "tasks/streams",