[3.14] gh-135944: Add a "Runtime Components" Section to the Execution Model Docs (gh-135945) (#139509)

gh-135944: Add a "Runtime Components" Section to the Execution Model Docs (gh-135945)

The section provides a brief overview of the Python runtime's execution environment.  It is meant to be implementation agnostic,
(cherry picked from commit 46a1f0a9ff6e7cbea111dcb35e7aced1a7a3f3dc)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>