Remove beats code generation and document Fields.java with JavaDoc

The beats/ folder contained YAML field specifications (Elastic Beats
format), Ruby/Python generation scripts, and generated asciidoc files
that were used to produce Fields.java and index template files.

- Delete cf-java-logging-support-core/beats/ entirely (YAML specs,
  scripts, generated asciidoc and index templates)
- Remove the `generate` Maven profile from cf-java-logging-support-core/
  pom.xml, which invoked the Ruby and Python scripts
- Replace the auto-generated header comment in Fields.java with full
  JavaDoc on every constant: type, required/optional, shared/log/request
  scope, description, and examples where applicable; constants are
  grouped into shared-context, application-log, and request-metrics
  sections
- Update README.md to link to Fields.java instead of the deleted beats
  folder

Signed-off-by: Karsten Schnitter <k.schnitter@sap.com>

Author: KarstenSchnitter

README.md

@@ -34,7 +34,8 @@ With such standardized formats in place, it becomes much easier to ingest, proce
 analysis stacks such as [ELK](https://www.elastic.co/webinars/introduction-elk-stack).
 
 If you're interested in the specifications of these standardized formats, you may want to have a closer look at the
-`fields.yml` files in the [beats folder](./cf-java-logging-support-core/beats).
+field name constants and their documentation in
+[`Fields.java`](./cf-java-logging-support-core/src/main/java/com/sap/hcp/cf/logging/common/Fields.java).
 
 While [logstash-logback-encoder](https://github.com/logstash/logstash-logback-encoder) is tied
 to [logback](http://logback.qos.ch/), we've tried to keep implementation neutral and have implemented the core