Doc: Break up source file for Logstash ECK docs into smaller files

[karenzone]

Proposal

Use case. Why is this important? Content for Logstash ECK continues to grow. Having a single source file is becoming increasingly unwieldy.

Restructure Logstash source file in preparation for addition of new content.

• Move Logstash content to a new folder, and break the monolithic source file into smaller units to align with approach used for Elasticsearch and Security content.
• After restructuring files, make sure that content builds and renders properly with no regressions or loss of content.

GitHub history

Ideally, we preserve GitHub attributions and history in the process. I need some guidance on how to approach this.

[karenzone]

Go forward plan:

• Rename original file to preserve history and attributions for the project.
• Manually add contributors to subsequent files.