Improve DirAccess page

**Your Godot version:**
4.1.1

**Issue description:**
The page begins with an example of using `get_next()` to get all the files in a directory. This could trick people into thinking that this is the *only* way to do this instead of being able to use `get_files()` and `get_files_at()`. In Godot 3 those methods did not exist so `get_next()` was the only way to get the files. I suggest that we also put an example with `get_files()` or `get_files_at()` at the top too.

Also, the `get_files()` method warns that only .import files will be returned instead of the actual image paths. However, it doesn't tell you what to do if you actually *want* to iterate through the res:// directory in an exported project. I suggest that we tell them that the resource loader will automatically detect the location of the resource from the file path if you remove the ".import" from it.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_diraccess.html