PEP 12: Add list of intersphinx mappings

PEP 12 is very helpful in writing PEPs and has a section on intersphinx. However, to know the available intersphinx mappings, one has to know where to look in the sphinx configuration.

I think it would be useful to include the list directly in PEP 12.

Alternatively, if that is considered to be at too high a risk for getting outdated, one could add an explanation on where to find it.

首先定位 PEP 12 以及 issue 中引用的 Sphinx 配置。确认 intersphinx 映射维护在哪里，然后使用该列表更新 PEP 12，或说明读者可以在哪里找到该列表；完成的标准是：无需预先了解配置知识即可发现可用的映射。