Link from typing docs to specific sections in the typing spec

The typing docs currently frequently link to the PEPs that defined new typing features. Instead, they should link to the typing spec, which unlike the PEPs is a living document.

In the online documentation, we can do this with intersphinx. I have this running locally and can send a PR.

I'm less sure about how to handle this in the docstrings. For example, Protocol's docstring refers to PEP 544: https://github.com/python/cpython/blob/b32789ccb91bbe43e88193f68b1364a8da6d9866/Lib/typing.py#L1987. Should we provide a full URL to the spec, like https://typing.readthedocs.io/en/latest/spec/protocol.html?

Linked PRs

• gh-117540

先檢查 typing 文件連結和 intersphinx 設定，接著查看 Lib/typing.py 中所參照的 PEP 544 連結附近的 Protocol 文件字串。決定線上文件和文件字串應如何指向現行的 typing 規範，並確認相關連結使用規範中預期的章節。