Nerdctl as a library (documentation and API work)

### What is the problem you're trying to solve

See https://github.com/containerd/nerdctl/discussions/4337 for context.

Building a simple cli that just runs a container is not trivial for newcomers, and certainly requires significant familiarity with nerdctl internals to get it to work.

Furthermore, the "API" is largely undocumented, and there are a number of extra things to implement on-top of using `pkg` (hooking of logging and oci-hooks).

### Describe the solution you'd like

Suggesting:
1. documentation
2. express a better / simpler API hiding away some of the complexity

### Additional context

_No response_