doc: check and update user guide

[musslick]

• [x] installation @chadcwilliams
• [x] theorists @chadcwilliams
• [x] experimentalists @chadcwilliams
• [x] experiment runners @chadcwilliams
• [ ] State @younesStrittmatter @hollandjg (include wrappers here)
• [ ] Workflow @younesStrittmatter
• [ ] Online "Behavioral" Experiments @younesStrittmatter

[musslick]

for each section, check the following:

• [ ] make sure everything is aligned with new terminology: https://docs.google.com/presentation/d/1OdP4M2ByW8GKTFvcCgDG5HDzUwHKdFEhj1ddH4ClvEw/edit#slide=id.g27e19158dea_1_0
• [ ] make sure it works
• [ ] no more pooler/sampler distinction
• [ ] make sure it conforms with the state logic (if it is relevant for the corresponding section)