Frontend: Separate GraphQL and REST into 2 pipeline mini graphs

We are separating the GraphQL and REST implementations of the pipeline mini graph into 2 independent directories so that:

• Anyone using or contributing to the PMG can understand clearly how to update the code
• We can easily remove the deprecated REST version once all apps have migrated over to GraphQL

This should also include a README.md in the pipeline mini graph directory to explain the differences and how to contribute.

Architectural documentation on the refactor for more info

Breaking the code out into work items so that MR reviews are comprehensive and efficient. I will update the task details as I add MRs.

Task	MR
Move legacy files to new directory	!154625 (merged)
Move remaining legacy pmg code to legacy directory	!154818 (merged)
Create README for PMG	!154964 (merged)