Docs: instruct user to put TQTC keyring in systemwide folder, not ~

The instructions for enabling our Debian package repos at https://doc.qt.io/qt-6/qt-debian-packages.html tell the user to put the GPG key in their home directory. This generally breaks invocations of `sudo apt-get update` because the user's home directory is generally not globally +x, as is required for the update process to access the keyring.

Recommend a change where the user is told to put the GPG key in /etc/apt/keyrings/tqtc instead.