docs: make the onboarding path work when followed literally

[Davidslv]

I followed the onboarding docs literally in fresh apps built from the published 0.2.0 gem. They didn't work as written.

What broke

Doc	Problem
Tutorial	"SQLite is fine". The migrations use jsonb and fail on SQLite (verified).
Tutorial	Sends readers to /auth/sign_up, which returns 404. The real path is /auth/registration/new.
Tutorial, Getting Started	Tell readers to add mount lines and include Auth::Authentication, which the generators already write.
All three	No bundle install between generators. Billing adds stripe, so the next generator fails to boot the app (verified: teams failed).
All three	No encryption keys step. Sign-up returns 500 with Missing Active Record encryption credential (verified).
All three	No tailwindcss:build after design --shell. Pages return 500 with The asset 'tailwind.css' was not found (verified).
Getting Started	Generated auth before core.
Engine Catalogue	The wiring example mounted Core at / (the generator uses /core), left out admin, and didn't say which includes are manual.

Verified after the fix

• Tutorial path: rails new --database=postgresql, bundle add seams, install, core, auth, design --shell, Tailwind build, encryption keys, migrate. Sign-up returns 302 and lands on the dashboard with 200.
• README quick start, all 7 engines with bundle install after each: migrate passes, bin/seams list shows 7 engines, sign-up works.
• The auth controllers skip_before_action :authenticate_identity! on sign-in, sign-up and password reset, so the catalogue's app-wide before_action advice is safe.
• The docs-site build passes. The README links now point at the live URL (davidslv.uk/seams; github.io redirects there).