Document administrators_authorized_keys file permission requirements

"OpenSSH for Windows" version: 8.1.0.1
Server OperatingSystem: Windows 11 Pro, 21H2 build 22000.675

The documentation at docs.microsoft.com refers to the administrators_authorized_keys file being used by sshd to authenticate user accounts that are members of the administrator group.

However, creating this file with proper public keys is not enough as it will inherit read permissions that must be removed before the file is accepted by sshd.

Plenty of people have been struggling with this, see fx https://stackoverflow.com/a/64868357/936466

I suggest improving the documentation at docs.microsoft.com.

An even better alternative would be to drop the administrators_authorized_keys concept entirely (discussed in #1324).

refs #1701

Start with the linked OpenSSH server configuration documentation and compare its administrators_authorized_keys guidance with the linked Windows setup instructions. Confirm the required file permissions and update the documentation to explain them clearly, including the commands or steps needed for sshd to accept the file.