Add PR action to validate notebook format

[brandonbaker-openai]

Summary

Adds a github action to validate the format of all changed notebook files

Motivation

Added to make sure only valid notebook changes are merged

Tests

1. With no .ipynb changes, confirmed the action passed trivially
2. Intentionally mangling a notebook (05f37b5), confirmed the workflow failed:

Run python .github/scripts/check_notebooks.py
Validating 1 notebook(s)...
examples/vector_databases/chroma/hyde-with-chroma-and-openai.ipynb: INVALID - Notebook does not appear to be JSON: '{\n  {\n   "cell_type": "markdown",\n  ...
1 invalid notebook(s) found.
Error: Process completed with exit code 1.

3. With a valid notebook edit (c0d0297), confirmed the workflow passed:

Run python .github/scripts/check_notebooks.py
Validating 1 notebook(s)...
All changed notebooks are valid.

[brandonbaker-openai]

Reached out to Katia for suggestions on additional approvers

Update: Connected offline and she said it looks good to her