Make X.D.Extending refer to X.H.StatusBar

[slotThe]

Description

Currently, the logHook section of X.D.Extending talks about X.H.DynamicLog quite extensively. Since at this point we heavily encourage users to migrate over to the X.H.StatusBar interface, it would be nice to rewrite this section to use that API instead.

Additionally, in accordance with https://github.com/xmonad/xmonad-contrib/pull/637, it would be neat to have these docs be a little bit more "advanced"—instead of telling users how to use X.H.StatusBar[^1] perhaps discuss a small part of the implementation instead.

[^1]: This should already be taken care of by the documentation itself, as well as the tutorial.