#401 Update Chapter on Value-Only Serialization - second changes

[BirgitBoss]

Chapter on Value-Only Serialization was updated: The number refers to https://github.com/admin-shell-io/aas-specs/issues/401#issuecomment-2051915440

• rules for non-submodels added (4)
• rules improved and corrected, added information about optional elements (4) and (8)
• schema fixed w.r.t optional elements (9)

part of https://github.com/admin-shell-io/aas-specs/issues/401