Portal: Remove unused pages, empty pages, Fix missing H1s (#7979)

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## PR-Codex overview
This PR primarily focuses on the deletion of several `.mdx` files related to the `pay` and `account` sections, while adding new content to the `nebula` API reference and making updates to the `engine` wallet configurations.

### Detailed summary
- Deleted multiple `.mdx` files in the `pay` and `account` sections.
- Added new API endpoints in `nebula`:
  - `Execute Action`
  - `Get Session`
  - `List Sessions`
  - `Clear Session`
  - `Create Session`
  - `Delete Session`
  - `Update Session`
- Updated headings for clarity in existing `.mdx` files related to wallet configurations.

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

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Author: MananTank

apps/portal/src/app/account/account-info/page.mdx

@@ -7,12 +7,12 @@ export const metadata = {
 };
 
 
-## AWS KMS wallet
+# AWS KMS wallet
 
 An AWS KMS Wallet is a wallet securely stored in your AWS account. Engine can create and transact with the wallet, but not delete it.
 
 <Callout variant="info" title="AWS KMS Support">
-  AWS KMS wallets are not currently supported on Engine Cloud. They are currently supported on Engine Dedicated v2, and will be available soon with Dedicated v3. 
+  AWS KMS wallets are not currently supported on Engine Cloud. They are currently supported on Engine Dedicated v2, and will be available soon with Dedicated v3.
 </Callout>
 
 

apps/portal/src/app/engine/v2/configure-wallets/aws-kms/page.mdx

@@ -6,7 +6,7 @@ export const metadata = {
         "Learn how to set up a Circle wallet with thirdweb Engine.",
 };
 
-## Circle Wallet
+# Circle Wallet
 
 [Circle Programmable Wallets](https://developers.circle.com/w3s/programmable-wallets) is a Wallet as a Service (WaaS) solution designed to simplify the creation and management of secure Web3 wallets and their private keys. Engine can create and transact with the wallet, but not delete it.
 

apps/portal/src/app/engine/v2/configure-wallets/circle-wallet/page.mdx

@@ -1,6 +1,6 @@
 import { Details } from "@doc";
 
-## Google Cloud KMS wallet
+# Google Cloud KMS wallet
 
 A Google Cloud KMS Wallet is a wallet securely stored in your Google Cloud account. Engine can create and transact with the wallet, but not delete it.
 

apps/portal/src/app/engine/v2/configure-wallets/gcp-kms/page.mdx

@@ -1,6 +1,6 @@
 import { Details } from "@doc";
 
-## Smart Server Wallets
+# Smart Server Wallets
 
 Smart server wallets are the recommended way to perform blockchain operations with Engine. Each smart server wallet consists of an EOA (managed internally by Engine) and a smart account (using thirdweb's default account implementation).
 

apps/portal/src/app/engine/v2/configure-wallets/server-wallet/page.mdx

@@ -7,7 +7,7 @@ export const metadata = createMetadata({
 		"Engine performs blockchain actions using server wallets that you own and manage.",
 });
 
-## Create a wallet
+# Create a wallet
 
 For AWS or Google Cloud KMS wallets, you must provide your credentials.
 

apps/portal/src/app/engine/v2/features/backend-wallets/page.mdx

@@ -1,11 +1,12 @@
 import { EndpointMetadata } from './EndpointMetadata';
 
-<EndpointMetadata />
 
 # Chat Actions
 
 Chat actions represent blockchain transactions or operations that Nebula has prepared in response to your request. The response includes both a detailed explanation in the `message` field and the actual transaction data in the `actions` array.
 
+<EndpointMetadata />
+
 **Example Response with Chat Action:**
 ```json
 {

apps/portal/src/app/nebula/api-reference/chat/page.mdx

@@ -1,3 +1,5 @@
 import { EndpointMetadata } from './EndpointMetadata';
 
+# Clear Session
+
 <EndpointMetadata />

apps/portal/src/app/nebula/api-reference/clear-session/page.mdx

@@ -1,3 +1,5 @@
 import { EndpointMetadata } from './EndpointMetadata';
 
+# Create Session
+
 <EndpointMetadata />

apps/portal/src/app/nebula/api-reference/create-session/page.mdx

@@ -1,3 +1,5 @@
 import { EndpointMetadata } from './EndpointMetadata';
 
+# Delete Session
+
 <EndpointMetadata />