[Docs] Remove Nebula from portal navigation (#7576)

```
<!--

## title your PR with this format: "[Portal] Feature: Remove Nebula from portal navigation"

## Notes for the reviewer

This PR removes all user-facing references to Nebula from the portal navigation and homepage, as requested.

-   **Key point:** All Nebula pages and underlying content remain untouched; only navigation links and homepage sections were removed.
-   Removed Nebula from the main header navigation and the APIs dropdown.
-   Removed Nebula sections from the homepage (API Reference and Learning Resources).
-   Updated the Vault section icon on the homepage from `NebulaIcon` (which was removed) to `ConnectIcon`.

## How to test

1.  Run the portal locally.
2.  Verify "Nebula" is no longer present in the main navigation bar or the "APIs" dropdown.
3.  On the homepage (`/`), confirm that the "Nebula" cards are removed from the "API Reference" and "Learning Resources" sections.
4.  Confirm that the "Vault" card in the "Learning Resources" section now uses the `ConnectIcon`.
5.  Run `npm run portal:build` to ensure the project builds successfully.

-->
```

---

[Slack Thread](https://thirdwebdev.slack.com/archives/C04DYC3G5NZ/p1752094214240499?thread_ts=1752094214.240499&cid=C04DYC3G5NZ)

<!-- start pr-codex -->

---

## PR-Codex overview
This PR primarily focuses on removing references to `Nebula` from the application, including links and UI components, while retaining and emphasizing `Insight` and `Vault`.

### Detailed summary
- Removed `Nebula` link from `connectLinks` in `Header.tsx`.
- Deleted `Nebula` API reference card from `ReferenceSection` in `page.tsx`.
- Eliminated `Nebula` card from `LearningResourcesSection` in `page.tsx`.

> ✨ Ask PR-Codex anything about this PR by commenting with `/codex {your question}`

<!-- end pr-codex -->

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

## Summary by CodeRabbit

* **New Features**
  * None.

* **Bug Fixes**
  * None.

* **Refactor**
  * Removed all references to the "Nebula" feature from navigation menus and resource sections.
  * Updated the icon for the "Vault" article card.

* **Chores**
  * Cleaned up unused imports related to "Nebula".

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

Author: saminacodes

apps/portal/src/app/Header.tsx

@@ -51,10 +51,6 @@ const links = [
     href: "/insight",
     name: "Insight",
   },
-  {
-    href: "/nebula",
-    name: "Nebula",
-  },
   {
     href: "/vault",
     name: "Vault",
@@ -127,10 +123,6 @@ export const connectLinks: Array<{
 ] as const;
 
 const apisLinks = [
-  {
-    href: "/nebula/api-reference",
-    name: "Nebula",
-  },
   {
     href: "https://insight.thirdweb.com/reference",
     name: "Insight",

apps/portal/src/app/page.tsx

@@ -14,7 +14,6 @@ import { BridgeIcon } from "../icons/products/BridgeIcon";
 import { ConnectIcon } from "../icons/products/ConnectIcon";
 import { EngineIcon } from "../icons/products/EngineIcon";
 import { InsightIcon } from "../icons/products/InsightIcon";
-import { NebulaIcon } from "../icons/products/NebulaIcon";
 import { PlaygroundIcon } from "../icons/products/PlaygroundIcon";
 import { cn } from "../lib/utils";
 import DocsHeroDark from "./_images/docs-hero-dark.png";
@@ -148,12 +147,6 @@ function ReferenceSection() {
           iconClassName="text-muted-foreground"
           title="Bundler"
         />
-        <SDKCard
-          href="/nebula/api-reference"
-          icon={NebulaIcon} // TODO: actual openAPI docs
-          iconClassName="text-muted-foreground"
-          title="Nebula"
-        />
       </Grid>
     </section>
   );
@@ -194,16 +187,10 @@ function LearningResourcesSection() {
           icon={InsightIcon}
           title="Insight"
         />
-        <ArticleCardIndex
-          description="API interface for LLMs"
-          href="/nebula"
-          icon={NebulaIcon}
-          title="Nebula"
-        />
         <ArticleCardIndex
           description="Non-custodial key management service"
           href="/vault"
-          icon={NebulaIcon}
+          icon={ConnectIcon}
           title="Vault"
         />
       </Grid>