Instruction for adding layer to QGIS needs to be updated.

We don't use XYZ to render tiles on QGIS anymore. Instead, we can use WMS/WMTS layer to show the layers.

I already created two new APIs to provide required data for QGIS. I also updated the popup window in MiSaC UI by replacing the old link with the new one. But the instruction in documentation should be aligned, too.