Document common usecases and recommended usage

(came up during OpenTelemetry C++ SIG)

Creating this issue to track the common use-cases that we want to document in more detail. It would be ideal to add this documentation to readthedocs/Getting Started so that these common users can get started quickly without having to fully understand all the build options.

We can start in the roles defined in the specification here https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/glossary.md#user-roles

I've interpreted these roles to map to the use-cases of:

• Application Owner - I want my application to generate telemetry, I need to use sdk (and possibly install a vendor-specific)
• Instrumentation Author - Any-one using the api to instrument their application OR library code
• Library Author - A library that is instrumented using the api but is intended to be used by other applications (or libraries) and may be distributed by itself
• Plugin Author - Someone (likely a vendor) who uses sdk and provides an SDK plugin that application owners can install

Please add comments for any use-cases that you may be interested in.

Start with the readthedocs Getting Started section and the user roles in the OpenTelemetry specification glossary. Review how the C++ API and SDK are currently explained for application owners, instrumentation authors, library authors, and plugin authors. Done means adding clear guidance for the identified common use cases without requiring readers to understand all build options.