Enable documentation for Instrumentor classes

[mauriciovasquezbernal]

Each integration should implement the _instrument and _uninstrument methods according to the BaseInstrumentor interface. Those methods are private and are not included in the documentation with the current configuration: https://opentelemetry-python.readthedocs.io/en/latest/ext/flask/flask.html#api.

We could enable them by using :private-members:, but probably it's too much and will include documentation of members we don't want to, another issue is that the documented name would include the underscore, and we want the user to call the method without it on the BaseInstrumentor class. Also, we don't have a way to document the parameters that those methods could take as they are declared as **kwargs.

[github-actions]

This issue was marked stale due to lack of activity. It will be closed in 30 days.

[codeboten]

This is done now, docs live here: https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/base/instrumentor.html