Add Cynthion Gateware Tutorials

[antoinevg]

This PR adds three two five Gateware tutorials of increasing complexity to the documentation:

1. Gateware Blinky - Ye olde blinky. Includes instructions for installing the FPGA toolchain and uploading bitstreams.
2. USB Gateware: Part 1 - Enumeration
3. USB Gateware: Part 2 - WCID Descriptors
4. USB Gateware: Part 3 - Control Transfers
5. USB Gateware: Part 4 - Bulk Transfers

NOTE: In the interests of releasing some tutorials sooner rather than later I'll be adding the USB Audio Class Device tutorial in a separate PR.

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Rendered: https://cynthion--197.org.readthedocs.build/en/197/

Depends on:

• Add support for using yowasp when yosys is not available. luna#283
• Add support for using yowasp when yosys is not available. luna-soc#36

[mndza]

This is great! 😃 Added some comments/suggestions.