chore(tracing): move tracing-actix-web examples into workspace

[robjtede]

Move the custom root span, OpenTelemetry, and request ID response header examples from actix-extras/tracing-actix-web/examples into tracing/.

Register all three examples as workspace members, use workspace package settings and Tokio local runtime entry points, and update the run instructions. Use tracing-actix-web 0.7.22 as the minimum version with OpenTelemetry 0.32 support. Expose the OTLP port in the custom root span example's Jaeger command. The existing Mainmatter workshop is unchanged.

The request ID example uses middleware::from_fn and examples_common::init_standard_logger(). The telemetry examples retain their custom subscribers for JSON console logs and OTLP export. They shut down their exporters with spawn_blocking so the local runtime can continue to drive I/O.

Validation:

• cargo clippy -p tracing-custom-root-span -p tracing-opentelemetry-example -p tracing-request-id-response-header --all-targets --all-features --offline
• cargo build -p tracing-custom-root-span -p tracing-opentelemetry-example -p tracing-request-id-response-header --offline
• HTTP smoke checks for all three new examples, including successful shutdown. Verified distinct request IDs, a request ID on a 404 response, and the custom greeting route.
• Scoped nightly rustfmt, Taplo format/lint, and Prettier checks passed.

Trace export to a running collector was not tested.

Companion source cleanup: actix/actix-extras#839. Merge this PR first so the updated documentation links resolve.