Revamp the Appenders page

[vy]

As agreed in the site structure document, revamp the Appenders page such that

• Outline:
  • What is an appender?
  • List appenders and link to the associated plugin reference page
  • Extending appenders
    • Mention about *Manager classes, how to handle restart
    • Explain that it is not a trivial undertaking
• Address
  • I found an error description in the document #1883
  • Document 2.x module/feature deprecation #1950
• Target 2.x, port to main

[ppkarwasz]

While restructuring the "Appenders" page we should explain the meaning of the immediateFlush option.
Looking at #2023, #2025 and other discussions, users often think that immediateFlush forces the OS to write the data to the underlying storage device.

That is not true, immediateFlush only flushes the Java buffers to the OS and due to the atomicity of the UNIX read()/write() calls this is usually enough for the users: if the appender logs to a pipe, the receiver should get messages up to PIPE_BUF atomically. For files and symbolic links there is no limit and operations should always be atomic.