Update the official tutorial with initial db migration

[chikamichi]

What do you want and why?

Hi. Following along the official tutorial, I stumbled upon an "error" where the User table would be missing, preventing from signing up.

I figured it out easily: one must run blitz prisma migrate dev.

Possible implementation(s)

Before the Sign up as a user section, add another section "explaining" the need for data (including a table of users), SQLite as a default, the concept of a migration, the schema.prisma file and more importantly, the migration command.

Additional context

Using latest Blitz version as of 02/08/2021: 0.38.6

[chikamichi]

Moved to blitz-js/blitzjs.com#532 🙏