Add changelog

[mihkeleidast]

Currently, the changelog is automatically generated in the code repo. Unfortunately, that is not very accessible when people are looking at the docs site. Additionally, the changelogs and GitHub release pages have a lot of unnecessary clutter (releases with no changes, example package releases, etc) that make it a lot harder to understand.

We should:

• add the changelog, even if updating is manual at first - we don't do that many releases and it would be easy, as it's mainly copy-pasting content from the main repo changelogs.
• figure out a way to automate adding the changelog to the docs site upon release. This should be easier once we get the main repo to a single-version setup, as there'll be a single changelog in the root that may be easy to copy over.