JS Docs: consolidate /api/tracing and /instrumentation

[cartermp]

We have some duplicated content in /api/tracing and /instrumentation. The latter is the most up to date, and the former was donated to this repository a while ago.

Both have a lot of views: https://play.honeycomb.io/opentelemetry-web/datasets/opentelemetry.io/result/fGMfroiQT5Z?useStackedGraphs

I propose that we migrate stuff from /api/tracing that's not in /instrumentation over to /instrumentation, remove the /api/tracing page, and then redirect /api/tracing to /instrumentation.

[cartermp]

I think that with the information we have in /tracing, this sample in particular could probably be obviated: https://opentelemetry.io/docs/instrumentation/js/api/tracing/#describing-a-span

Although we don't show how to use context.with today, it's a rather advanced way to propagate context and create child spans with startActiveSpan gets the job done.

[cartermp]

Closin'