[FLINK-11292] [doc] Move predefined TypeSerializerSnapshot class docs to a top-level subsection
[FLINK-11292] [doc] Move predefined TypeSerializerSnapshot class docs to a top-level subsection

This commit adds an independent, top-level subsection named "Predefined
convenient TypeSerializerSnapshot classes" to the "Custom Serialization
for Managed State" page.

This section covers details about using the SimpleTypeSerializerSnapshot
and CompositeTypeSerializerSnapshot classes, what they do, and in which
cases they are suitable to be used by a serializer.

This closes #7475.

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