Revamp the `Configuration` page

[vy]

Due to the fast development lifecycle of Log4j 2, the configuration options documented in the configuration page and those effectively available in the code are currently out-of-sync.

The purpose of this task is to:

• check the documentation of Log4j System Properties with their usage in the code,
• if they differ, fix either the documentation page or the code,
• simplify and restructure the page.

Configuration page outline

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

• Outline:
  • Supported formats
    • General structure of configuration file
    • How do formats map to each other?
  • Basics
    • Very briefly explain appender, logger, etc.
    • Refer to the “Architecture” page for details
  • Properties
    • Mention about fuzzy matching
    • List them in definition list (for main artifacts)
  • Recipes
    • Examples for common use cases
  • Links to the “Guides” page contents
  • Programmatic configuration
• Fix LOG4J2-3684
• Target 2.x, port to main

[ppkarwasz]

The following "documentation" problems were fixed by adapting the code to the documented behavior:

• Deprecate log4j2.defaultStatusLevel property #2481,
• Prevent log4j2.isWebapp from overriding other constants #2517,
• Improves support of log4j2.enableThreadlocals property #2550

[ppkarwasz]

The following documentation changes were applied:

• Revamp configuration properties #2552
• Revamp Configuration page #2574