docs: removes `npx` info & moves src link up (#4972)
docs: removes `npx` info & moves src link up (#4972)

FIXES: #4970 

confirmed rest of instructions work so left them in, even though duplicate of guide over at Material UI. thoughts?
JeromeFitz/next.js,BlancheXu/test,BlancheXu/test,flybayer/next.js,zeit/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,BlancheXu/test,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js