Add powershell to list of requirements on windows

Author: aeisenberg

docs/codeql/codeql-overview/system-requirements.rst

@@ -26,6 +26,10 @@ For extraction of compiled languages (C/C++, C#, Go, Java) and Ruby on Linux:
 - ``glibc`` version 2.17 or greater must be installed.
 - ``musl-c``-based Linux distributions, such as Alpine Linux, are not supported.
 
+For extraction using autobuild on Windows:
+
+- The ``PowerShell.exe`` executable must be available on the ``PATH``.
+
 For TypeScript extraction on all platforms:
 
 - Node.js 14 or higher must be installed and available on the ``PATH`` as ``node``.