Move to markdown-it-py markdown parser implementation

Mainly implemented in #107

This releases moves to the markdown-it-py markdown parser implementation, concurrently with myst-parser. Additionally:

• Add notebook render tests
• Add simple solution for reporting correct cell index/line number:
  Report line number as <cell index>*10000 + <line number>. This is a simple solution to addresses #71, that doesn't require any complex overrides of the sphinx reporting machinery.
• Make tests use the actual sphinx Application
• Re-write validation of which docs to execute/cache:
  Rather than having a global variable, we save the excluded paths in the sphinx env and use a separate function is_valid_exec_file. Also added tests