Specify where `RenderGraph` is in Migration Guide, add it to `bevy_render::prelude` (#24265)

# Objective

- Fixes #24257

## Solution

- `RenderGraph` is already pub, but it moved locations. Specify where in
the migration guide.
- Added `RenderGraph` to `bevy_render`’s prelude. This can be removed if
desired.

## Testing

- ci