Improve documentation

[pricem]

This issue will serve as a running commentary on documentation tasks:

• [x] Provide an overview of the system with a couple high level diagrams
• [x] Rearrange notes/writeup.txt into something more useful
• [ ] Locate photos and link to them appropriately
• [ ] Compile assorted plots of measured performance
• [x] Write up detailed description of modular architecture and interfaces
• [ ] Write up detailed description of boards (v2 modules, carrier, clock, isolator)
• [x] Write up detailed description of digital logic architecture
• [ ] Write up software documentation - how to configure MPD, Python scripts, etc.
• [ ] Compile subjective impressions of audio quality from various iterations
• [ ] Add Gerber files and organize assembly BOM spreadsheets

[pricem]

Initial documentation is pushed and synced to da-platform.readthedocs.io. I'll continue working down the list as time permits.

[pricem]

Requested by passive420:

Please post some pics of your PCB designs and modules, the pics in the documents are gone...