Add popup/tooltip messages to graphics

[StephanStS]

The graphics can be extended by an image tooltip.

![<name_displayed_during_image_load>](<link_to_image> "<image_tooltip>"){: <image properties>}

![Pi-hole web interface screenshot](../assets/images/dietpi-software-dnsserver-pihole.png){: width="500" height="410" loading="lazy"}
![Pi-hole web interface screenshot](../assets/images/dietpi-software-dnsserver-pihole.png "Pi-hole v6 screenshot"){: width="626" height="517" loading="lazy"}