Elixir Security - Spawning External Executables

We should continue rounding out what is included from the EEF's research into the module for Elixir Security and add the "Spawning External Executables" lesson.

Relevant Resource:
- https://erlef.github.io/security-wg/secure_coding_and_deployment_hardening/external_executables

首先查看现有的 Elixir Security 课程结构以及链接的 EEF“Spawning External Executables”资源。按照周边材料的相同格式添加课程，并确认新主题已包含在课程中且准确反映该资源。

由索引模型根据 Issue 内容生成。