[3.13] gh-124872: Refine contextvars documentation (GH-124773)

[rhansen]

• 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.

(cherry-picked from commit 9940093)

• Issue: Replace context enter and exit events with single "context switched" event #124872

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