Add more details to docs

[gdalle]

In order to be able to merge #1343 with the restructuring that was already done, here's a record of the things we still want to add:

• pointer shenanigans for Duplicated(x, dx) with typeof(x) != typeof(dx) => Add FAQ section to docs #1343 (comment)
• discussion on sparse and structured arrays => Add FAQ section to docs #1343 (comment)
• GPU and friends => Add FAQ section to docs #1343 (comment)

[wsmoses]

Closing as PR's have landed