Refresh Log4j API Kotlin Documentation

[u-ways]

Hello,

This is just a documentation refresh to match the latest logging-log4j2 project conventions.

Changelog

• Add missing CONTRIBUTING.md and CODE_OF_CONDUCT.md
• Update deadlinks, impove upon documentation
• Add a simple usage example to match logging-log4j2
• Add missing Github PR contributor guidelines to match logging-log4j2
• Add Maven Central Badge
• Fix Build status badge

Trivial changes - No functionality changes. Feel free to amend as needed. :)

[u-ways]

@jvz Can you have a look please?