[Portal] updated billing docs (#6477)

CORE-0000

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## PR-Codex overview
This PR updates the billing information section in the documentation, specifically enhancing the instructions for viewing past invoices and modifying the related content.

### Detailed summary
- Updated the navigation steps for viewing past invoices from **Settings > Billing > Manage Billing** to **Settings > Invoices**.
- Changed the instructions for viewing invoices to emphasize breakdown by plan and usage.
- Removed an unused `invoices-settings.png` import.

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Author: saminacodes

apps/portal/src/app/account/billing/assets/invoices-settings.png

@@ -2,8 +2,6 @@ import { DocImage } from "@doc";
 import billingSettings from "../assets/billing-settings.png";
 import invoicesSettings from "../assets/invoices-settings.png";
 
-
-
 import { createMetadata } from "@doc";
 
 export const metadata = createMetadata({
@@ -35,8 +33,7 @@ To update your billing information:
 
 To view your past invoices:
 
-1. On your dashboard, navigate to **Settings > Billing > Manage Billing**.
-2. In the Stripe Customer Portal, scroll to the **Invoice History** section.
-3. View or download your past invoices.
+1. On your dashboard, navigate to **Settings > Invoices**
+2. View or download past invoices to see breakdown by plan and usage.
 
 <DocImage src={invoicesSettings} />

apps/portal/src/app/account/billing/manage-billing/page.mdx

@@ -30,5 +30,4 @@ Invite your team members using email addresses and assign a member or owner role
 
 </Step>
 
-</Steps>
-
+</Steps>