Auto-publish ePub GitHub action

[PhilStollery]

Hi Merlin,

I was listening to Reconcilably Differentiated and thought, I might know how to do that. For my current work I use GitLab - but CI/CD pipelines seem similar.

This PR adds a GitHub action that on commit builds an epub. You should edit it to only run when a change to wisdom.md is made - and the epub is rubbish and missing content (but I think you know how to use pandoc to fix that).

But this is a bare bones start for you if you want to use it. Just something i could quickly do in 10 mins while I should be doing real work.