Change "environments" to "frameworks"

This is more consistent with the language we're using at Stackbit, and it's more generally understood and consistent with the docs inside the section.

Address #33 first so that moving these isn't a problem. (But still put in redirects.)

按要求，先处理 issue #33。然后使用“environments”定位网站内容，将术语替换为“frameworks”，并确定必须保留的重定向。相关部分及其引用使用新措辞且不破坏现有 URL，即表示完成。

由索引模型根据 Issue 内容生成。