Add Lightning, Worker and CLI changelogs to the doc site

[josephjclark]

This has been at the back of my mind for years (I'm sure there's a kit issue).

But we should add to docs.openfn.org the changelogs for key products, so that users can get a sense of what is changing and when.

There's currently a changelog at https://openfn.github.io/lightning/changelog.html but I don't know how users are supposed to access that

I know changelogs are a bit technical but that's OK. And I'd love a culture of making sure that changelogs are human readable and useful!