How to make a player rotate in a direction with a slope of the ground.

Hi, I'm making my first game, but I don't know how to make a player rotate on the same slope as the ground. Sample-(I use rigid body)

I think it can be done with the help of "freeze Rotation", so I got it, but a new problem arose - if I approach the edge of the slope, then the player falls and turns in an unfamiliar direction - either sideways, upside down or backwards . This is not a bad thing until you start moving with the WSAD, because if a player has turned in any other direction, if I press W, then he goes to where the falling has turned-

The code I use-

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 public class PlayerMovement : MonoBehaviour
 {
     public Rigidbody rigg;
     private Vector3 moveDirection;
     public float speed;
     void Start()
     {
         rigg = GetComponent<Rigidbody>();
     }
     void Update()
     {
         float hor = Input.GetAxisRaw("Horizontal");
         float ver = Input.GetAxisRaw("Vertical");
         moveDirection = transform.forward * ver + transform.right * hor;
         rigg.AddForce(moveDirection.normalized * speed, ForceMode.Acceleration);
     }
 }

Thanks!