Document setting manual span parent on website

[dyladan]

Takeaway here I guess is to add documentation to the website

Originally posted by @dyladan in #1940 (comment)

The user was having a hard time finding a good example on how to use context, setSpan, and so forth to manually parent a span. Better documentation on the website would have been a big help.

[dyladan]

This was the example which eventually helped https://github.com/open-telemetry/opentelemetry-js/blob/main/examples/basic-tracer-node/index.js

[pizzaz93]

Would https://opentelemetry.io/docs/instrumentation/js/instrumentation/#create-nested-spans be this code example documented?

[github-actions]

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[dyladan]

Website docs just had a major overhaul