[docs-beta] Add testable code snippets (#23836)

This moves our beta docsite code examples into its own folder under
`examples`. See instructions for how to run and test this.

Local testing, buildkite.

NOCHANGELOG

examples/docs_beta_snippets/tox.ini

@@ -12,6 +12,7 @@ install_command = uv pip install {opts} {packages}
 deps =
   source: -e ../../python_modules/dagster[test]
   source: -e ../../python_modules/dagster-pipes
+  pypi: dagster[test]
   -e ../../python_modules/dagster-graphql
   -e ../../python_modules/dagster-webserver
   -e ../../python_modules/libraries/dagster-duckdb-pandas