Create a Using a logging API guide

[vy]

As agreed in the site structure document, create a Using a logging API guide such that

• Move the content from api-separation.adoc and remove the latter
• Refer to the Concepts page
• Address concerns in Website: API Separation - "Using Log4j2 as an SLF4J Implementation" is outdated/misleading (SLF4J particularly) #1460 too
• Address log4j-api: Incomplete/unclear description (simple implementation) #1474 and mention that log4j-api contains a built-in implementation too
• Target 2.x, port to main

[vy]

Superseded by #2535.