replace create_seed_checkpoint.md with a note in docs/checkpoint.md

[tianyu-l]

Stack from ghstack (oldest at bottom):

• [BE] add integration test for the generation script #741
• [BE] restructure tests and assets folders #740
• [BE] dump compile trace to CI output for debugging, and reduce uploaded folder size in CI #739
• -> replace create_seed_checkpoint.md with a note in docs/checkpoint.md #736

This PR does the following

1. remove create_seed_checkpoint.sh as it's basically run_llama_train.sh with some config, so we still keep the useful functionality
2. fix the error in setting DTensor random seeds when creating a seed checkpoint by checking if spmd_mesh.get_coordinate() is not None
3. add documentation in docs/checkpoint.md on how to generate one
4. remove its use in CI as PP can work without one