Revamp the `API` page

[vy]

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

• Revamp the content
• Link from/to 5min
• Address following tickets:
  • Website: API Separation - "Using Log4j2 as an SLF4J Implementation" is outdated/misleading (SLF4J particularly) #1460
  • Revamp the manual introduction #1468 (make it visible in the manual intro) (3.x ca978bf)
  • Website: Manual should be more prominent in left sidebar #1469 (make it visible in the menu)
  • (doc) Clarify the usage of parameterized logging in Log4j 2 API page #1918
• Make api.adoc the central entry point (merge api-separation.adoc into there)
• Cover content from:
  • flowtracing.adoc -> (2.x: f1de65f, 28268e8), (3.x: 43181c2)
  • markers.adoc -> rewrote Markers #2584 (Ported to 3.x with 891c08e)
  • messages.adoc -> Rewrote Messages #2591
  • eventlogging.adoc -> (2.x: 9ea0223), (3.x: dac0ba8)
  • logbuilder.adoc -> rewrote logbuilder #2595
• Target 2.x, port to main