Fix snippets_test.py::test_can_run_docs_code_snippets

[pavoljuhas]

Describe the issue

Problem: Unit test snippets_test.py::test_can_run_docs_code_snippets is supposed to verify Python code snippets in markdown files under the docs directory, but it looks for them in the dev_tools directory instead. Markdown files in dev_tools have no snippets to check so the test is a no-op.

$ pytest --quiet --co dev_tools/snippets_test.py::test_can_run_docs_code_snippets
dev_tools/snippets_test.py::test_can_run_docs_code_snippets[cirq-infra/README.md]
dev_tools/snippets_test.py::test_can_run_docs_code_snippets[triage-party/README.md]
dev_tools/snippets_test.py::test_can_run_docs_code_snippets[ci/README.md]

At the time of its writing in #2307 the snippets_test.py was in the docs directory, but #3227 moved it to dev_tools without adjusting its markdown files search.

Solution: Fix markdown files search in snippets_test.py and fix any issues that pop out in the docs/**/*.md files (some snippets may need to be excluded if it is too much hassle to make them pass).

Tell us the version of Cirq where this happens

1.7.0.dev0 at 53b552c