Add explanation for D/T markers to cache marker page

[Lineflyer]

Add explanation for D/T markers to cache marker page

[ztNFny]

Attached are 3 examples to explain how it works, imho that's better suited for the docs than the full set of 18 half-circles (actually there are 20, there's also the 0,5 step in case some connector ever uses that).

For the documentation it should be noted that some connectors are using ratings >5, those are shown as 5

[Lineflyer]

Done:
https://manual.cgeo.org/en/cachemarkers