docs: complete README structural parity with the Java and Python SDKs

The README was restructured to the sibling SDKs' shape **only where Go already had content** — the inline Hello World moved to `docs/core-quickstart.md`, the stale `doctoc` TOC was dropped, and the hub link was added. See `docs/adr/0001-canonical-documentation-ia.md`.

Sections still missing relative to [Java](https://github.com/conductor-oss/java-sdk/blob/main/README.md) (221 lines) and [Python](https://github.com/conductor-oss/python-sdk/blob/main/README.md) (186 lines), because they are net-new prose rather than a move:

- [ ] **Choose your path** — decision table routing readers to the right quickstart
- [ ] **Choose your Conductor server** — Orkes Developer Edition (recommended), Conductor CLI, Docker fallback
- [ ] **Why Conductor?** — positioning section
- [ ] **Common tasks** — task-oriented index
- [ ] **Troubleshooting** — top failure modes with resolutions
- [ ] **Support and project policies**

Match the siblings' heading text exactly so the three READMEs are navigable as one set. Note that Java's version omits the agent sections that do not apply to Go.

Start with the Go README and compare its headings with the linked Java and Python SDK READMEs, using docs/adr/0001-canonical-documentation-ia.md for the intended structure. Add the six listed sections with the siblings' exact heading text, omitting Java-only agent material, and confirm the README is structurally navigable alongside both siblings.

Written by the indexing model from the issue text.