Update distribution into sections and chapters

[rkharsan]

Currently the tutorial is organized in a somewhat peculiar way:

Looking at this, you would assume that "Drawing a Triangle", "Vertex Buffers", and "Depth Buffering" will be roughly equal in length or complexity. I assumed that all of these were chapters, roughly similar in length. But in reality, "Drawing a Triangle" alone is half the length of the entire tutorial. (To check this, see the PDF versions from https://vulkan-tutorial.com)

Drawing a Triangle also has 2 levels of nesting, while there are some top-level items with one (ex. Vertex Buffers) or no (ex. Depth Buffering) levels of nesting. I think it would be a good idea to organize things a bit differently to divide the tutorial into equal sized pieces.

[SaschaWillems]

Reworking that structure would require a large rewrite of the tutorial. We'll partially address this in #61, but the general structure will stay this way.