Add detailed documentation for the Db Migrator in Single Layer Web Application template

docs/en/solution-templates/single-layer-web-application/db-migrator.md

@@ -18,4 +18,61 @@ Unlike the Layered solution template, the Single Layer solution template does no
 
 ![Single Layer Solution: Db Migrator](images/single-layer-db-migrator.png)
 
-After the migration completes, a message will appear in the console. You can verify the success of the migration by checking the database.  
+After the migration completes, a message will appear in the console. You can verify the success of the migration by checking the database.
+
+## Database Migration Service
+
+Under the `Data` folder of the project, the `BookstoreDbMigrationService` class is responsible for database migration and seed data operations. The `MigrateAsync` method is called in the `Program` class to migrate the database when the application starts with the `--migrate-database` argument.
+
+First, it checks if the database is created and applies the pending migrations. Then, it seeds the initial data using the `SeedAsync` method. 
+
+```csharp
+public async Task MigrateAsync()
+{
+    var initialMigrationAdded = AddInitialMigrationIfNotExist();
+
+    if (initialMigrationAdded)
+    {
+        return;
+    }
+
+    Logger.LogInformation("Started database migrations...");
+
+    await MigrateDatabaseSchemaAsync();
+    await SeedDataAsync();
+
+    Logger.LogInformation($"Successfully completed host database migrations.");
+
+    var tenants = await _tenantRepository.GetListAsync(includeDetails: true);
+
+    var migratedDatabaseSchemas = new HashSet<string>();
+    foreach (var tenant in tenants)
+    {
+        using (_currentTenant.Change(tenant.Id))
+        {
+            if (tenant.ConnectionStrings.Any())
+            {
+                var tenantConnectionStrings = tenant.ConnectionStrings
+                    .Select(x => x.Value)
+                    .ToList();
+
+                if (!migratedDatabaseSchemas.IsSupersetOf(tenantConnectionStrings))
+                {
+                    await MigrateDatabaseSchemaAsync(tenant);
+
+                    migratedDatabaseSchemas.AddIfNotContains(tenantConnectionStrings);
+                }
+            }
+
+            await SeedDataAsync(tenant);
+        }
+
+        Logger.LogInformation($"Successfully completed {tenant.Name} tenant database migrations.");
+    }
+
+    Logger.LogInformation("Successfully completed all database migrations.");
+    Logger.LogInformation("You can safely end this process...");
+}
+```
+
+The `BookstoreDbSchemaMigrator` class is used in the `MigrateDatabaseSchemaAsync` method for the database migration process. It is responsible for applying migrations to the database.