[Code health] Update README files to reflect new Nx usages

[gino-m]

Commands to run the root of the project:

• For convenience, devs should run npm i -g nx during initial setup.
• Run npm i once in the root to install deps as before.
• nx lint, nx affected:lint, nx test, and nx affected:test do what you think. Do you can also just nx lint web, nx test functions, etc.
• Local emu flow: run nx emulate, then nx serve web in a separate terminal.
• To run local dev server at http://localhost:4200/ against staging (ex-dev Firebase): nx run web:serve:staging (or nx serve web -c staging .
• To deploy to staging (ex-dev): nx deploy:staging.

Note the new configuration names:

• local --> development (*default)
• dev --> staging
• prod --> production

Also note that there's now no need to build each package in the monorepo independently; Nx tracks deps (see nx graph) and builds required packages as needed.