Add new "API Reference" documentation section

This issue can be closed by adding a new "API Reference" section under the top-level "CUDA C++ Core Libraries" that unifies existing API documentation under a structure more like the following:

```
CUDA Object Model

Memory Management

General Utilities
Error Handling
Hash Functions
Pair -Tuple/Optional/Expected/Variant/Bitset

Containers

Algorithms
Parallel
Cooperative
Sequential
Numerics
Concurrency support
Iterators
Metaprogramming Library
Chrono
Input/Output Library
```

As a concrete small step forward, this issue will intentionally *not* change any of the other doc structure.