[MNY-163] Portal: Fix broken links in payments sidebar (#8021)

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## PR-Codex overview
This PR focuses on updating the `sidebar` configuration in two files: `sidebar.tsx` for the bridge and payments sections. It removes an entry and modifies the `href` properties for navigation links to ensure correct routing.

### Detailed summary
- Removed the `Playground` entry from the `sidebar` in `apps/portal/src/app/bridge/sidebar.tsx`.
- Updated the `href` for the `Playground` entry in `apps/portal/src/app/payments/sidebar.tsx` to point to `/payments`.
- Changed `href` values for `Sell Tokens`, `Swap Tokens`, `Get Token Prices`, and `Get Routes` to use absolute paths.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

* **Bug Fixes**
  * Corrected Payments sidebar destinations: Playground now points to the dedicated payments playground, and Bridge-related items use absolute paths for reliable, consistent navigation across the app.

* **Chores**
  * Removed the obsolete Playground link from the Bridge sidebar to streamline navigation.
  * Cleaned up an unused icon import.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: MananTank

apps/portal/src/app/bridge/sidebar.tsx

@@ -1,4 +1,4 @@
-import { ExternalLinkIcon, ZapIcon } from "lucide-react";
+import { ZapIcon } from "lucide-react";
 import type { SideBar } from "@/components/Layouts/DocLayout";
 import { EngineIcon, ReactIcon, TypeScriptIcon, UnityIcon } from "@/icons";
 import { UnrealEngineIcon } from "../../icons/sdks/UnrealEngineIcon";
@@ -12,11 +12,6 @@ export const sidebar: SideBar = {
       name: "Get Started",
       icon: <ZapIcon />,
     },
-    {
-      href: "https://playground.thirdweb.com/",
-      icon: <ExternalLinkIcon />,
-      name: "Playground",
-    },
     { separator: true },
     {
       isCollapsible: false,

apps/portal/src/app/payments/sidebar.tsx

@@ -13,7 +13,7 @@ export const sidebar: SideBar = {
       icon: <ZapIcon />,
     },
     {
-      href: "https://playground.thirdweb.com/",
+      href: "https://playground.thirdweb.com/payments",
       icon: <ExternalLinkIcon />,
       name: "Playground",
     },
@@ -38,19 +38,19 @@ export const sidebar: SideBar = {
           name: "Send a Payment",
         },
         {
-          href: `bridge/sell`,
+          href: `/bridge/sell`,
           name: "Sell Tokens",
         },
         {
-          href: `bridge/swap`,
+          href: `/bridge/swap`,
           name: "Swap Tokens",
         },
         {
-          href: `bridge/tokens`,
+          href: `/bridge/tokens`,
           name: "Get Token Prices",
         },
         {
-          href: `bridge/routes`,
+          href: `/bridge/routes`,
           name: "Get Routes",
         },
         {