Improve "Simple tilemap example"

[BrettWitty]

Following the request 1 in #163, I updated the example to use simpler defaults and the current API.

Changes include:

• Using the default Tilemap to avoid calling spawn_chunk_at_point later.
• Explained the use of HandleId::random::<TextureAtlas>
• Put the tilemap up top and texture atlas work a little lower.
• Expanded out Tile and explained all the data
• Used insert_tile rather than set_tile

[joshuajbouw]

Looks good to me.

[dwoznicki]

Just FYI, I think these lines

    // Which tile from the TextureAtlas
    sprite_index = 0,
    
    // Which z-layer in the Tilemap (0-up)
    sprite_order = 0,

should probably be

    // Which tile from the TextureAtlas
    sprite_index: 0,
    
    // Which z-layer in the Tilemap (0-up)
    sprite_order: 0,

And function tilemap.set_default_texture_atlas does not seem to exist for me. Perhaps it should be tilemap.set_texture_atlas?

[BrettWitty]

Yep. My bad. I'll make the changes and push them.