Document different types of splashscreens

[Eeems]

It would be useful for users to know what types of splashscreens there are in the guide portion.

In the development portion, we should document the formats of the files.

In the tech articles, we should have an article about creating your own splashscreens.

[Eeems]

This should also document the difference in how startup and low battery screens are handled between the rM1 and rM2, as they are displayed by uboot, not xochitl or remarkable-shutdown. Thus, they are different formats between the devices due to the display mechanisms.