docs: remove stale table of contents entries in adding-pages guide (#9204)

The table of contents still linked to "Learn Page Frontmatter",
"Frontmatter Fields" and "Accessible Components" sections that no
longer exist in the document, producing three dead anchor links.
Remove the stale entries so every TOC link resolves again.

Signed-off-by: ump45nose <1729065730@qq.com>

Author: ump45nose

docs/adding-pages.md

@@ -11,15 +11,12 @@ This guide explains how to create new pages and content for the Node.js website.
   - [4. Update Navigation (if needed)](#4-update-navigation-if-needed)
 - [Adding Learn Pages](#adding-learn-pages)
   - [Learn Page Structure](#learn-page-structure)
-  - [Learn Page Frontmatter](#learn-page-frontmatter)
-    - [Frontmatter Fields](#frontmatter-fields)
   - [Update Learn Navigation](#update-learn-navigation)
   - [Add Translation Keys](#add-translation-keys)
 - [Content Guidelines](#content-guidelines)
   - [Markdown Features](#markdown-features)
   - [Code Blocks](#code-blocks)
   - [Multiple Code Tabs](#multiple-code-tabs)
-  - [Accessible Components](#accessible-components)
 - [File Organization](#file-organization)
   - [Content Structure](#content-structure)
   - [Asset Management](#asset-management)