Improve usability of layouts

The following issues have been sources of confusion for our first layout user:

1. Having to assign a position in order to assign a span is confusing.
2. Putting whitespace into a cell isn't documented anywhere.
3. Providing a column_span that would extend a cell beyond the end of the grid doesn't throw an error.

I'm also going to create a new doc section that contains examples and images of the grids they produce.

Also, the following bugs have been discovered during testing in TOPAZ:

1. Calling add_child(html.Div()) triggers a double render of the div, even inside of the GridLayout context.
2. Alignment parameter isn't behaving as expected for HBoxLayout.
3. Children of a VBoxLayout inside of an HBoxLayout don't stretch correctly in flexbox.