Add cooperative groups

[neon60]

Add cooperative groups documentation which contains:

• Reference
• How to
• Tutorial

[neon60]

I don't think we should have two files in the docs folder named the same, such as how-to/cooperative_groups.rst and reference/cooperative_groups.rst, even though they are currently in separate folders.

I think one should be renamed.

Renamed.

[neon60]

@lpaoletti ready for the another round of review.