Update Alignment Handbook example

[peterschmidt85]

• [Docs] Reworked README.md to move the focus from Alignment Handbook to how it can be used with dstack
• [Hotfix] Removed tensorboard from config.yaml (due to the non-fixed issue with Alignment Handbook) throwing an exception
• [Bugfix] Replaced use_flash_attention_2: true with attn_implementation: flash_attention_2
• [Bugfix] Added bnb_4bit_quant_storage: bfloat16 (otherwise Alignment Handbook threw an exception)
• [UX] Added fleet configurations for both single-node and multi-node tasks
• [UX] Remove the logic of coping config files (e.g. ACCEL_CONFIG_PATH)
• [UX] Remove the unnecessary environment variables (FT_MODEL_CONFIG_PATH and ACCEL_CONFIG_PATH)
• [UX] Pin the version of Python to 3.10
• [UX] Moved ACCELERATE_LOG_LEVEL to env
• [UX] Changed 24GB:2 to 24GB for simplicity
• [UX] Changed nodes: 3 to nodes: 2 for simplicity
• [UX] Added comments to task configurations