01-tutorials - Add Dash0 as partner observability integration

**Which component is impacted by this improvement?**
- [ ] 01-AgentCore-runtime
- [ ] 02-AgentCore-gateway
- [ ] 03-AgentCore-identity
- [ ] 04-AgentCore-memory
- [ ] 05-AgentCore-tools
- [x] 06-AgentCore-observability
- [ ] 07-AgentCore-E2E

**Improvement Description**

Add Dash0 as a third-party observability option in two places:

- `01-tutorials/06-AgentCore-observability/04-Agentcore-runtime-partner-observability/Dash0/` — Jupyter notebook demonstrating AgentCore Runtime + Strands Agents + Dash0 using OTLP/HTTP export
- `03-integrations/observability/dash0/` — standalone Python example using uv with a Strands travel agent

Dash0 is an OpenTelemetry-native observability platform. The integration uses OTLP/HTTP (port 4318) with Bearer token authentication, matching the same pattern as the existing Datadog and Instana integrations.

Also updates READMEs in `04-Agentcore-runtime-partner-observability/`, `06-AgentCore-observability/`, and `03-integrations/` to list Dash0 alongside the other supported platforms.

I have a draft ready and can send a PR once confirmed.