Integrate docs with codebase

This task is to make the meltano.com project obsolete and have it fully integrated with Meltano.

Provide more detailed instructions on why certain commands are being used (in the Release section on CHANGELOG)

Improve RELEASE instructions

Clarify understanding on changelog-cli "changelog-cli does actually infer the next version number from the actual changes in UNRELEASED, that's the whole point of using it in the first place. If an MR includes a break change, then the majoir version will automatically get bumped. new will bump minor and changes only bump patch."