File organisation

Currently, there are 4 parts of docs:

• API: contains detailed docs about API designs. (Note that system_design.md is a temp draft for discussing high-level design, not for API definitations).

• knowledges: contains theorical knowledges/designs for the project (mainly diffusion stuffs).

• notes4coding: contains deatiled description for coding, may be useful to coding agents (like codex).

• papers: contains useful papers we've found in pdf.