Documentation: clarify cxx::bridge for std::vector<Type*> bindings

[pkarneliuk]

Describe the bug
Documentation (https://google.github.io/autocxx/workflow.html) does not explain how to implement the #[cxx::bridge] for passing vector of pointers to objects std::vector<Type*> from C++ to Rust. This is relatively common use-case.

To Reproduce
autocxx can not generate bindings for std::vector of pointers to objects, like std::vector<Type*>

instrument.hpp

// instrument.hpp
#pragma once
#include <vector>

struct Instrument {
    int id;
    Instrument(int i) : id{i} {}
    virtual ~Instrument() = default;
    virtual int get() const {
        return id;
    }

    static Instrument* create(int i) {
        return new Instrument(i);
    } 
};

std::vector<Instrument*> instrument_list() {
    std::vector<Instrument*> instruments = {
        Instrument::create(1),
        Instrument::create(2),
        Instrument::create(3)
    };
    return instruments;
}

lib.rs

// lib.rs
use autocxx::prelude::*;

include_cpp! {
    #include "instrument.hpp"
    generate!("Instrument")
    generate!("instrument_list")
}

Generated .rs contains:

# [doc = "autocxx bindings couldn't be generated: Type std::vector was parameterized over something complex which we don't yet support"] 
pub struct instrument_list ; 

Expected behavior
It would be great to provide some example of #[cxx::bridge] when C++ code pass std::vector<Type*> to Rust code and methods of Type can be called from Rust side.

Additional context
There is the unit test test_vector_of_pointers() for similar use-case, but there is no example how to write correct #[cxx::bridge] for passing vector of pointers to objects.

[adetaylor]

Fundamentally, autocxx is limited to the types supported by cxx, and I don't believe that a CxxVector<*const T> is supported by cxx.

About three years ago I tried to implement this but ran out of time - feel free to take over that work! Once it's supported in cxx, it should be easy enough to support in autocxx too.