Add pooled buffer serialization overloads

[JKamsker]

Summary

• add Memory<byte> and IBufferWriter<byte> serialization overloads in BsonSerializer to reuse caller-provided buffers without extra allocations
• verify round-tripping via ArrayBufferWriter<byte> and pooled buffers with new integration tests
• document the API in the README and add the System.Memory dependency needed for netstandard builds

Testing

• dotnet build LiteDB.sln
• dotnet test LiteDB.Tests/LiteDB.Tests.csproj -f net8.0

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https://chatgpt.com/codex/tasks/task_e_68cfd6f5cd78832ab3a6d54ea97f0040