…blebooks#102)

* added functionality of jupyter-cache
* writing tests to cover the all/most cases
* adding execution and cacheing documentation
* moved fixtures to conftest

* Add development documentation

The previous implementation fails when an existing notebook is changed with failing code

…o `.ipynb` (executablebooks#109)

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

…s#107)

This commit move to 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 executablebooks#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 exclude paths in the sphinx env and use a seperate function `is_valid_exec_file`. Also added tests