docs: add documentation on ContextManagers and Zone.js (#386)

## Which problem is this PR solving?

Adds documentation on supplying your own context manager

References:
- https://opentelemetry.io/docs/languages/js/context/#context-manager
-
https://github.com/open-telemetry/opentelemetry-js/tree/main/packages/opentelemetry-context-zone-peer-dep#installation
- open-telemetry/opentelemetry-js#1502
- open-telemetry/opentelemetry-js#5104
- open-telemetry/opentelemetry-js#3030

## How to verify that this has the expected result
read the docs