Automate eBook release workflow

[zoul]

It would be nice to release new ebook version when we push updates to the source Markdown. What needs to be done:

• [x] Convert Markdown to eBook using Pandoc
• [x] Pandoc only supports EPUB, could we convert that into MOBI, too?
• [x] Add our custom cover image?
• [ ] How about bundling our custom font for headlines?
• [x] Could we add the current version or a time stamp into the colophon? (It’s already embedded in the source Markdown document.)
• [x] Should the action run on every push? (No, just when a release tag gets pushed.)
• [x] Mint new GitHub release? (Sort of, already done in the current workflow, but just a draft of the release is created.)
• [x] Should we introduce a changelog and list the changes since last release automatically? (Nice to have, but don’t know how at the moment, so we fill the release notes for each release by hand.)

[zoul]

This is good for now, formatting improvements can be tackled in a separate issue.