Qbs needs a section about design philosophy in the documentation

We should have a section in the documentation that talks about the design philosophy of Qbs - what its goals are, what its goals AREN'T, and what its strengths and weaknesses are. A comparison to make (and makefile generators in general) should also be given (more in depth, beyond the current single sentence "qbs is an all in one tool..." that we currently have).

The introduction section of the ninja manual is a good place for inspiration: https://ninja-build.org/manual.html#_introduction

Comparison to Ninja would also be useful so people don't think it is faster than Qbs  (at least in the ways that matter, given that they have different design goals).