This refactors `extract_code_example` to return a nested list
of code snippets between '{code_example|start/end}' instead of
returing a list of all the lines between '{code_example|start/end}'
markers in the code examples.
Appropriate changes have been made to render_python_code_example.
Zulip's openapi specification in zulip.yaml has various examples
for various schemas. Validate the example with their respective
schemas to ensure that all the examples are schematically correct.
Part of #14100.
Previously api_description and api_code_examples were two independent
markdown extensions for displaying OpenAPI content used in the same
places. We combine them into a single markdown extension (with two
processors) and move them to the openapi folder to make the codebase
more readable and better group the openapi code in the same place.
