docs(migrations): add documentation for RunMigrations

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internal/pkg/migrations/migrations.go

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 var ErrDatabaseEnvironmentVariableNotSet = errors.New("DB_URL environment variable not set")
 
+// RunMigrations applies database schema migrations to a PostgreSQL database
+// using the specified migration source. It loads environment variables from a
+// predefined file and checks for the "DB_URL" variable (trimming any
+// surrounding quotes). If "DB_URL" is missing, it returns
+// [ErrDatabaseEnvironmentVariableNotSet]. It then opens a PostgreSQL driver
+// connection, initializes a migration instance with the given source, and
+// executes the migration using the Up method, returning any errors encountered
+// except for those indicating no changes.
 func RunMigrations(source string) error {
 	env, _ := gotenv.LoadEnvFromFS(fs.EnvFile, gotenv.LoadOptions{OverrideExistingVars: false})