gh-124872: Refine contextvars documentation (#124773)

* Add definitions for "context", "current context", and "context
    management protocol".
  * Update related definitions to be consistent with the new
    definitions.
  * Restructure the documentation for the `contextvars.Context` class
    to prepare for adding context manager support, and for consistency
    with the definitions.
  * Use `testcode` and `testoutput` to test the `Context.run` example.
  * Expand the documentation for the `Py_CONTEXT_EVENT_ENTER` and
    `Py_CONTEXT_EVENT_EXIT` events to clarify and to prepare for
    planned changes.