Refactor description of span kind

[lmolkova]

Fixes #3172

(Built on top of #4088)

Changes

• Explains kinds without assuming presence of parent/children
• Adds links as another correlation mechanism
• Normalizes nested client spans even further - database, messaging, RPC, and LLM semantic conventions require CLIENT kind for logical client operation.
• Does not touch INTERNAL kind yet - What's INTERNAL span kind and when it should be used #4179

• Related issues Clarify semantic of SpanKind regarding parent/child relationships #3172, DB, messaging, gRPC (over HTTP): clarify nested client spans  semantic-conventions#674, Instrumentation layers and suppressing duplicates oteps#172, Allow INTERNAL GenAI/db spans instead of requiring the kind to be CLIENT semantic-conventions#1315
• [ ] Related OTEP(s) #
• [ ] Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• [ ] spec-compliance-matrix.md updated if necessary

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