Added block diagram to Overview page

[bhs128]

Added a png and svg version of a system block diagram. Rewrote/expanded the Overview page and included the diagram along with a more broad description of the project and design choices. Open to any feedback/suggestions.

[mooinglemur]

When adding binaries directly to a git repo, I would highly encourage optimizing those for size. If the file is replaced, the old file remains as baggage in the repo's history, and binaries can't benefit from incremental storage in the same way as text. A few hundred k here or there can add up.

The PNG in this PR can easily be better optimized. A simple pngcrush saves 41% space on cx16_block_diagram.png, and even more can likely be saved with different conversion options. I would hope to see it under 100k if it can still retain good fidelity.

[mooinglemur]

Thanks!