Address caveats katalog update script

We need to address (some of) the current caveats in the catalog update script:

• move script from /hack to /tools/python
• add developer documentation
• clone the katalog repo into a temp directory instead of relying on the user to have done it prior to running the script
• not all of the YAML files in the katalog repo may want to be included in the catalog upload, so alert the user to all the changes instead of relying on git diff
• read the catalog_upload.json file prior to regenerating it to capture assets that live outside the katalog repo and re-add them later

Originally posted by @ckadner in https://github.com/machine-learning-exchange/mlx/issues/200#issuecomment-926195710