Update the official tutorial with initial db migration #2621

### What do you want and why?

Hi. Following along the [official tutorial](https://blitzjs.com/docs/tutorial#sign-up-as-a-user), 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

Start at the linked official tutorial and inspect the section on signing up as a user, including the steps that precede it. Add a section before it explaining the initial data, SQLite, schema.prisma, migrations, and `blitz prisma migrate dev`; the tutorial is done when readers know to run the migration before signing up.

Written by the indexing model from the issue text.