Add design section to documentation

[j2kun]

Add a Design section to the documentation website with the following pages.

The purpose of this would be to ramp up someone who already knows a decent amount of MLIR, and a decent amount of FHE, but is not familiar with our particular design choices, or might want to slice and dice our passes for their own purposes.

• [x] secret
• [ ] tfhe/cggi pipeline
• [x] "HECO" pipeline
• [ ] polynomial abstraction/math
• [ ] dialect taxonomy/diagram
• [ ] exit/entry dialects

[asraa]

I can start a Design section on secret today since I'm working on secret.separator at the moment. It'll just be a start / placeholder and we can add more as we go

[j2kun]

I'm going to start on the secret docs because it's what I have the energy to do right now.