Update documentation for PEP 691

[gsnedders]

Describe the bug
The documentation claims the simple API is legacy (c.f. https://warehouse.pypa.io/api-reference/legacy.html)

Expected behavior
The documentation matches the implementation, which no longer considers the simple API as legacy (c.f. #11676). Additionally, it should document the PEP 691 implementation, rather than pushing people to use the (non-standard?) legacy JSON API.

To Reproduce
Load the documentation.

My Platform
N/A

Additional context

[woodruffw]

#16541 should address this: we now consistently refer to the 503/691 APIs as the "Index APIs" or "simple APIs."