[doc] Align agents logs documentation

[SylvainJuge]

Historically, the only log-related feature of the agents was log correlation, but this is no longer the case and it makes sense to create a more general Logs section next to Metrics.

Also, we now have a dedicated page in observability docs : https://www.elastic.co/guide/en/observability/master/application-logs.html that provides a good overview of all the possible ingestion strategies.

Proposal (for each agent)

• create a Logs section next to Metrics, move the existing Log correlation one into it and create redirect if necessary
• Logs section structure:
  • listing the agent log features, example in Java: https://www.elastic.co/guide/en/apm/agent/java/master/logs.html#logs
  • add a small paragraph and a link about ecs-logging, while not an agent feature it is definitely related, example in Python: https://www.elastic.co/guide/en/apm/agent/python/master/log-correlation.html#ecs-logging
  • one section dedicated for each log-related feature (the moved Log correlation being one of them), exemple in Java https://www.elastic.co/guide/en/apm/agent/java/master/logs.html#logs

Also, this is a good opportunity to document features that are present but not yet documented, for example log reformatting is supported in python and ruby and is not yet documented.

This is the final step to #765

Checklist

• Go [doc] Align agents logs documentation apm-agent-go#1465
• Java [doc] Align agents logs documentation apm-agent-java#3149
• .NET [doc] Align agents logs documentation apm-agent-dotnet#2110
• Node.js [doc] Align agents logs documentation apm-agent-nodejs#3399
• Python [doc] Align agents logs documentation apm-agent-python#1837
• Ruby [doc] Align agents logs documentation apm-agent-ruby#1394
• update https://www.elastic.co/guide/en/observability/master/application-logs.html with new links to respective agents
  • Log correlation
  • Log reformatting
  • Update links for APM agents logs observability-docs#3055
  • dotnet, nodejs and ruby agents links will be covered once they have been released.