Update migration instructions for payment providers

[CarlSargunar]

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I've followed the Umbraco Documentation Style Guide and can confirm that:

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📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[CarlSargunar]

Additional changes to add : Vendr.Services.CurrencyService.GetCurrency(ctx.Order.CurrencyId); to be replaced with injected ICurrencyService

Vendr.Services.CountryService.GetCountry(ctx.Order.PaymentInfo.CountryId.Value) to be replaced with injected ICountryService

[CarlSargunar]

And IVendrBuilder to IUmbracoCommerceBuilder

[CarlSargunar]

Also mention about ModelsBuilder - eg Homepage node has a reference to Store, but the namespaces no longer exist.

	///<summary>
	/// Store
	///</summary>
	[global::System.CodeDom.Compiler.GeneratedCodeAttribute("Umbraco.ModelsBuilder.Embedded", "10.8.11+d8f68d2")]
	[global::System.Diagnostics.CodeAnalysis.MaybeNull]
	[ImplementPropertyType("store")]
	public virtual global::Umbraco.Commerce.Core.Models.StoreReadOnly Store => this.Value<global::Umbraco.Commerce.Core.Models.StoreReadOnly>(_publishedValueFallback, "store");

[CarlSargunar]

Changes to Order.AddProduct, writeableOrder.AddProduct