feat: Add a basic changelog mechanism

[bsmth]

Description:

First proper try at keeping a relevant changelog for admins. There's a bit of manual work at the moment, but it's high importance for surfacing changes to readers and will be good to iterate on.

I was considering a few different ways of tracking changes, using git tagging, keeping branches per release train etc, for now, the simplest approach is to prefix changelog entries in a single CHANGELOG.md. We may want to revisit in future.

Motivation:

Keep changes visible and improve traceability.

Related issues and pull requests:

• Change tracking / versioning #83

[bsmth]

@mkaply do you want to give this a try with

npm run build && npm run preview

Then: http://localhost:4324/changelog/

[bsmth]

I'm going to merge in this state and pending feedback we can asjust