Documentation hub - Improve navigation

Moved and updated the following documentation to streamline the left-hand navigation.

• Moved the following sections into the Accept payments category:
  • Manage payment methods
  • Checkout
  • Payment Links
  • Payments API
  • Payment extensions
  • Card payments
  • Recurring payments
• Moved the Transactions section into the Manage business operations category and renamed the page to View and refund transactions.
• Moved the Dashboard and Settlement and billing sections into the Manage business operations category.
• Renamed the Reference materials category to Developer tools to better reflect its content.
• Moved the Sandbox section into the Developer tools category.
• Created a new Resources section and moved the payments reference materials and best practices into this category.