Improve data types page

[rgommers]

This is largely a reorganization to improve the readability of this page, by replacing many subheadings with tables, by removing duplication in notes, and by reorganizing content.

Furthermore, the description on how dtype objects are used is extended:

• add more ways in which dtype objects are used
• explain that dtype= keywords determine output dtype and not internal/intermediate calculation dtype of arrays
• cross-link type promotion rules

The end result is that the page length is ~25% shorter and the sidebar navigation ~8x shorter, while adding some useful content.

[leofang]

Didn't review in detail but I love tables!