Correct migration step in phx.gen.auth docs

[jrko]

After running mix phx.gen.auth you are prompted to run mix deps.get followed by mix ecto.migrate.

The docs reference mix ecto.setup however, which seems to be deprecated in favor of mix ecto.create, neither of which are correct in this context, assuming a basic Phoenix app is up and running prior to adding auth.

This PR corrects the docs, replacing mix ecto.setup with mix ecto.migrate along with an appropriate description of the step.

It also adds the assumption that the reader has gone through the introductory guides and has a running Phoenix app, as well as calling out the addition of the Register and Log in links added to the default page.

I also took the liberty to remove some extraneous language about umbrella app root locations, which seemed unnecessary in the context of learning auth basics.

[josevalim]

ecto.setup is generated as part of new applications, so we should continue to refer to it:

phoenix/installer/templates/phx_single/mix.exs

Line 79 in a2e7de1

"ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],

.

If your app does not have it, maybe it was generated in older versions or removed?

[jrko]

Aah, I do indeed have ecto.setup (using Elixir 1.14.0, Erlang/OTP 24, Phoenix 1.7.14). I think my confusion came from a couple places:

1. The console output of mix phx.new and the Up and Running docs both reference ecto.create.
2. Searching Ecto HexDocs for ecto.setup returns no results, but ecto.create does.

I see now that ecto.setup is a valid alias in Phoenix, though it still seems a bit awkward to have the console output of phx.gen.auth reference ecto.migrate as a next step, while the auth docs reference ecto.setup.

Anyway, feel free to close the PR if you like. I was able to work through the confusion without too much fuss, but thought I might be able to streamline the docs a bit for future newcomers, like myself!

[josevalim]

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