[docs] CHANGELOG: keepachangelog format

[ianlewis]

Update CHANGELOG.md to be based off of https://keepachangelog.com/en/1.1.0/

We will need slight modifications so that we can logically group updates by builder but we can use the Added, Changed, etc. labels that are noted here.

Heading titles should also be formatted to avoid duplication (this will make it so the rendered html ids/url fragments can be permanent and don't change over time.

Something like:

## X.Y.Z

### X.Y.Z: Go builder

- **Added**: Added new input.
- **Changed**: Feature X was changed.