Update vault documentation text and faq (#8367)

```
[Portal] Fix: Update Vault open-source status and add FAQ

## Notes for the reviewer

This PR updates the Vault portal documentation to accurately reflect its open-source status.

*   Removed "open-source" from the main description and metadata on the Vault overview page.
*   Added a new FAQ entry under `/vault/faqs` clarifying that Vault is not yet open source.

## How to test

1.  Navigate to <https://portal.thirdweb.com/vault>
    *   Verify the main description no longer includes "open-source".
    *   (Optional) Inspect page metadata to confirm "open-source" is removed from the description.
2.  Navigate to <https://portal.thirdweb.com/vault/faqs>
    *   Verify the new FAQ "Is Vault open source?" is present with the answer "Vault is not yet open source, but we plan to release it soon."
```

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[Slack Thread](https://thirdwebdev.slack.com/archives/C09DS2CKGP2/p1762411784588169?thread_ts=1762411784.588169&cid=C09DS2CKGP2)

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## PR-Codex overview
This PR focuses on updating the description of the `Vault` service to remove references to it being open source and refining the language for clarity.

### Detailed summary
- Added a new `Details` section in `apps/portal/src/app/vault/faqs/page.mdx` about Vault's open-source status.
- Updated the `description` in `apps/portal/src/app/vault/layout.tsx` to remove "open-source."
- Revised the `description` in `apps/portal/src/app/vault/page.mdx` to remove "open-source."
- Modified the main description of `Vault` in `apps/portal/src/app/vault/page.mdx` for clarity.

> ✨ 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

## Documentation
* Added new FAQ entry addressing open-source availability with information on future release plans
* Updated Vault service descriptions emphasizing non-custodial key management design and TEE security architecture
* Refined documentation metadata to provide clearer communication of Vault's core features, security model, and service capabilities

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

Author: Yash094

apps/portal/src/app/vault/faqs/page.mdx

@@ -44,3 +44,7 @@ The admin key is temporarily stored in dashboard when performing actions that re
 Please note, you should never share your admin key with third parties.
 </Details>
 
+<Details summary="Is Vault open source?">
+Vault is not yet open source, but we plan to release it soon.
+</Details>
+

apps/portal/src/app/vault/layout.tsx

@@ -12,7 +12,7 @@ export default async function Layout(props: { children: React.ReactNode }) {
 
 export const metadata = createMetadata({
   description:
-    "Vault is an open-source non-custodial key management service, secured with TEE architecture (AWS Nitro Enclaves) and designed for blockchain applications.",
+    "Vault is a non-custodial key management service secured with TEE architecture (AWS Nitro Enclaves), designed for blockchain applications.",
   title: "Vault",
   image: {
     icon: "vault",

apps/portal/src/app/vault/page.mdx

@@ -5,7 +5,7 @@ import { ArrowLeftRightIcon, UserLockIcon, UsersIcon, WalletIcon } from "lucide-
 
 export const metadata = createMetadata({
 	title: "thirdweb Vault",
-	description: "An open-source, secure key management service for storing, managing, and protecting cryptographic keys and secrets.",
+	description: "A secure key management service for storing, managing, and protecting cryptographic keys and secrets.",
     image: {
         icon: "vault",
         title: "thirdweb Vault",
@@ -14,7 +14,7 @@ export const metadata = createMetadata({
 
 # Vault
 
-Vault is an open-source non-custodial key management service, secured with TEE architecture (AWS Nitro Enclaves) and designed for blockchain applications.
+Vault is a non-custodial key management service secured with TEE architecture (AWS Nitro Enclaves), designed for blockchain applications.
 
 ## Features