CI: Add workflow to automatically render all Example Notebooks

[s3alfisc]

To avoid any breaking API changes, we would like to add a github action to render all example notebooks.

In more detail, we'd like to render the following notebooks:

• compare_fixest_pyfixest.ipynb
• Difference-in-Differences Estimation
• Marginaleffects
• Quickstart
• Replicating the Effect
• Regression Tables / Etable

@juanitorduz fyi =)