rst-backticks: Smarter checks to ignore simple backticks inside rst code blocks

[DiddiLeija]

See https://github.com/pypa/pip/pull/10914, especially https://github.com/pypa/pip/pull/10914#issuecomment-1046557599.

We have Markdown stuff inside an RST code block, and there're Markdown backticks in there. However, the rst-backticks hook thinks those backticks are part of the doc (not the code block), so it fails. Can this check be smarter to avoid checking code blocks?