Enable publishing of changelog entries from this repo

_Once again, github has discarded the long description I wrote in the issue form. Lets type it out again..._

We currently publish changelog entries on the website via the website repo. This causes a disconnect between a feature being merged here and something being published on the website.

This task is to streamline the process.

One proposal is to have the website pull changelog entries from this repo (as it does docs already). That would allow a PR that implements a feature also include the changelog entry.

Some open points to figure out:

- Currently the website pulls from the `maintenance` branch. That means do we don't publish docs for something we haven't deployed to production yet - but it means we have to backport docs changes to get them live. Need to work out the workflow of what gets pulled from where - and when.
- We would still want to be able to publish changelog posts in the website repo directly for things that aren't in this repo (device agent, dashboard etc).