cubicweb #4291287 book: update the sphinx theme [validation pending]

make it a bit more consistant and moderm, use the new logo, etc.

prioritynormal
typeenhancement
done in3.20.0
load0.500
load left0.000
closed by#5e3fbdb035a8 [book] new theme based on pyramid theme (closes #4291287)
patch[book] new theme based on pyramid theme (closes #4291287) [applied]