Write Sip explaining `interpret`

We should explain how/why we do automatic language parsing based on extensions. Ref gulpjs/gulp#2073

Start by locating the website's existing documentation for `interpret` and read the referenced gulpjs/gulp#2073 issue. The work is done when the documentation explains how and why automatic language parsing based on file extensions works, including the relevant context from that reference.

由索引模型根据 Issue 内容生成。