No automatic date added

[mishaschwartz]

Previously the date_added field was automatically added to the registry the first time that the update.py script was run for that node. This was done by checking whether the registry already had the date_added field for the given node and adding one if it was missing.

However, when executing the update.py script in the CI workflow, we read the registry from the main branch and apply the changes to the current_registry branch. This means that the date_added field is never added to the main branch, which means that the update.py script re-adds the date_added field every time.

This PR makes the date_added field into a manually added field that should be included when a new node is added. This adds another (very small) manual step when adding a new node but with the current CI configuration there's no good way to do this automatically anymore.

I've also updated the schema so it should be obvious when the date_added information is missing. This means we won't forget to add this info when adding new nodes.

[alexandercyu]

No issues with me

[dchandan]

Looks good!