docs: split examples into read-only basic and guarded smoke

The previous examples/basic.py ran destructive operations (configure_set,
configure_delete, generate, reset, config_file_load) by default.  That is
not what a public 'basic' example should do.

- examples/basic.py: rewritten as a read-only example
  (retrieve_show_config, show, retrieve_return_values), uses a robust
  env-bool parser, prints response.result via a small helper, and uses
  the supported public import 'from pyvyos import ApiResponse, VyDevice'.
- examples/integration_smoke.py: renamed from the old basic.py, keeps
  the mutating operations, and refuses to run unless
  PYVYOS_ALLOW_MUTATING_EXAMPLE=1 is set.
- README: link both examples and describe what each one does.

Note: the dev/ note about a future Docker-based e2e harness is tracked
in the unreleased issue drafts; it is not part of this release.

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