Add PC BSA and BBR links to Windows on Arm overview

Add two Arm developer.arm.com architecture standard links to the
External resources section of the Windows on Arm overview page:
- PC Base System Architecture (PC BSA)
- Base Boot Requirements (BBR)

Co-authored-by: Copilot <[email protected]>

Author: GrantMeStrength

landing/arm-docs/overview.md

@@ -128,3 +128,5 @@ Our guidance to [Add Arm support to your Windows app](add-arm-support.md) walks
 - [Developer.arm.com: Port applications to Windows on Arm](https://developer.arm.com/documentation/102341/0400/Overview)
 - [Developer.arm.com: Building a Native Windows on Arm App with WinUI 3](https://developer.arm.com/documentation/102767/0100/)
 - [Developer.arm.com: Building libraries for Windows on Arm](https://developer.arm.com/documentation/102528/)
+- [Developer.arm.com: PC Base System Architecture (PC BSA)](https://developer.arm.com/Architectures/System%20Standards)
+- [Developer.arm.com: Base Boot Requirements (BBR)](https://developer.arm.com/Architectures/Base%20Boot%20Requirements)