update: first update for serialization documentation restructuring

[daniCsorbaJB]

This PR is for the first batch of documentation updates for KT-79623 [Docs][Libraries] Serialization introduction

[pdvrieze]

You want to be accurate in how serialization works. The "intermediate data" representation is misleading, an intermediate format is fundamentally impossible (even without supporting format specific escape hatches such as JsonPolymorphicSerializer)

[pdvrieze]

It may be worthwhile to clarify more that the serialization library provides the format agnostic infrastructure. Then the -json and -cbor libraries are format implementations (they are fundamentally different and are independent even though first-party).

[pdvrieze]

I added some further comments. It may be that some of them are better used in other pages (not in this request)