Document `2.x` module/feature deprecation

[ppkarwasz]

We should document both in code (Javadoc) and documentation the deprecation of the following 2.x modules or features.

• log4j-cassandra,
• CouchDB appender,
• GELF appender,
• Kafka appender,
• JeroMQ appender,
• log4j-jpa,
• log4j-mongodb3,
• log4j-spring-boot,
• Java EE SMTP appender,
• log4j-taglib,
• JsonLayout,
• YamlLayout.