Refactor IPdfPageImageRenderer registration

Encapsulated the registration of IPdfPageImageRenderer into a new
AddDocInfrastructure extension method in the DependencyInjection
class within the EnvelopeGenerator.Infrastructure.Doc namespace.

Updated Program.cs to use the AddDocInfrastructure method, improving
modularity and centralizing the service registration logic.

EnvelopeGenerator.Infrastructure.Doc/DependencyInjection.cs

@@ -0,0 +1,18 @@
+using EnvelopeGenerator.Application.Common.Interfaces.Doc;
+using Microsoft.Extensions.DependencyInjection;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace EnvelopeGenerator.Infrastructure.Doc;
+
+public static class DependencyInjection
+{
+    public static IServiceCollection AddDocInfrastructure(this IServiceCollection services)
+    {
+        services.AddScoped<IPdfPageImageRenderer, PdfPageImageRenderer>();
+        return services;
+    }
+}