Add logging to OpenTelemetry with Serilog

[maartenba]

Relates to https://github.com/orgs/DuendeSoftware/discussions/451#discussioncomment-15453156

• Add explanation and sample code with Serilog in https://docs.duendesoftware.com/identityserver/diagnostics/otel/#logs
• Add clarification about this in https://docs.duendesoftware.com/identityserver/diagnostics/logging/ (perhaps a note that calls out to the above?)