Reinstate `format.sh` and make `pre-commit` installation simpler (#15890)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

.pre-commit-config.yaml

+default_install_hook_types:
+  - pre-commit
+  - commit-msg
 default_stages:
   - pre-commit # Run locally
   - manual # Run in CI

format.sh

+#!/bin/bash
+
+echo "vLLM linting system has been moved from format.sh to pre-commit hooks."
+echo "Please run 'pip install -r requirements/lint.txt', followed by"
+echo "'pre-commit install' to install the pre-commit hooks."
+echo "Then linters will run automatically before each commit."
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