Docs: Improve Logger Extra Keys samples

[sliedig]

What were you searching in the docs?

ATM the extra keys samples do not show correct implementation. Shows Logger.AppendKeys method not Logger.AppendKey.

Is this related to an existing documentation section?

https://awslabs.github.io/aws-lambda-powertools-dotnet/core/logging/#extra-keys

How can we improve?

Add `Logger.AppendKey` sample
Add `Logger.LogInformation(extra keys...)` sample
Add anonymous type to log entire object payload

Got a suggestion in mind?

No response

Acknowledgment

• I understand the final update might be different from my proposed suggestion, or refused.