restructured, rewrote section 3

handout/sections/1c-hardcoded-information.typ

handout/sections/3-intro.typ

handout/sections/3a-client-server-communication.typ

handout/sections/3a1-credentials.typ

handout/sections/3a2-communication.typ

handout/sections/3b-webservices.typ

handout/sections/3c-server.typ

handout/sections/3d-excel.typ