docs: improve database migrations guide (#30914)

examples/database/employees/supabase/.gitignore

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+# Supabase
+.branches
+.temp
+.env

examples/database/employees/supabase/config.toml

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+# For detailed configuration reference documentation, visit:
+# https://supabase.com/docs/guides/local-development/cli/config
+# A string used to distinguish different Supabase projects on the same host. Defaults to the
+# working directory name when running `supabase init`.
+project_id = "employees"
+
+[db]
+# Port to use for the local database URL.
+port = 54322
+# Port used by db diff command to initialize the shadow database.
+shadow_port = 54320
+# The database major version to use. This has to be the same as your remote database's. Run `SHOW
+# server_version;` on the remote database to check.
+major_version = 15
+
+[db.seed]
+# If enabled, seeds the database after migrations during a db reset.
+enabled = true
+# Specifies an ordered list of seed files to load during db reset.
+# Supports glob patterns relative to supabase directory: `./seeds/*.sql'
+sql_paths = ['./seed.sql']

examples/database/employees/supabase/migrations/20241205075911_create_employees_table.sql

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+create table if not exists employees (
+  id bigint primary key generated always as identity,
+  name text not null,
+  email text,
+  created_at timestamptz default now()
+);

examples/database/employees/supabase/migrations/20241205080043_add_department_to_employees_table.sql

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+alter table if exists public.employees
+add department text default 'Hooli';

examples/database/employees/supabase/seed.sql

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+insert into public.employees
+  (name)
+values
+  ('Erlich Bachman'),
+  ('Richard Hendricks'),
+  ('Monica Hall');