[Step 1/3] Copy code-interpreter-custom to prompt-agents folder

[ericsuh]

We want to move the code-interpreter-custom docs to a prompt agent folder, because the example actually uses prompt agents rather than hosted agents, so the current hosted-agents location is incorrect.

Because this example is referenced by public documentation, we want to move these docs in three steps:

1. Copy this to a new location (that's this PR)
2. Update the documentation to the new location
3. Delete the old location.