docs: add chained Bash hook example

[yudin-s]

Summary

• Add a conservative PreToolUse hook example that blocks chained Bash commands outside quoted strings
• Document why Bash permission hooks should avoid unsafe auto-approval for compound shell commands
• Point plugin developers to a fail-closed mitigation while native compound-command permission parsing is unresolved

Addresses #16561.

Test plan

• python3 -m py_compile examples/hooks/block_chained_bash_commands_example.py
• smoke-tested simple, quoted, and chained Bash hook inputs
• git diff --check