Recover requirement's simplification from the motivation and move it to the overview

[kysrpex]

In commit 56e5e6b, part of release 2.4.0, the motivation section was transformed into a fundamental concept's section. Much content was added, but some was removed.

One of the removed contents is the mention on the fact that Requirement simplification was one of the motivations behind SimPhoNy. This information may now fit in the overview, so we should consider recovering it (using Git's history) and moving it there.