[docs] The relation between source type and rendering type needs to be clarified.

Nobody has claimed this yet.

In mapbox-style-spec, we have `source types` as:
```
vector
geojson
raster
image
video
canvas
```

And then we have `rendering type` as:
```
fill
fill-extrusion
line
symbol
circle
heatmap
raster
```

However, the docs did not say a souce type can be rendered as wich rending type, or a rendering type should specify which type of source. For example, how to add a layer that have a image\video\canvas type.

I conclude as below:

I think the docs should clarify this relation.

Start by locating the mapbox-style-spec documentation for source types and rendering types. Document which rendering types accept each source type, including image, video, and canvas, and describe how to add layers for them. Done means the supported relationships and usage guidance are explicit.

Written by the indexing model from the issue text.