Document cfg features

[Chris-Hawblitzel]

We should probably add documentation about the various --cfg features that you can apply when building Verus libraries, particularly when building directly with cargo/rustc. In this discussion, I want to list these features to help document them later:

• verus_keep_ghost
• verus_keep_ghost_body
• alloc
• allocator
• std

Also, we have features when building Verus itself:

• singular