feat: add script to generate ebook

[FineFindus]

Closes #1070.

I wanted to read this book on my e-reader, so I added a quick script, to generate an e-book from the markdown, using pandoc. This is not perfect, the issues include:

• Some of the used syntax breaks the e-book, so it needs to be removed
• Links, that link to the website (e.g. all links starting with @) do not work. Preferably, they would link to the chapter in the book, but I'm not sure how to do that.
• The layout is often imperfect, code blocks or text diagrams may be split over pages
• No real differentiating between posts
• New chapters do not start on a new page
• probably another dozen of things I didn't notice

I'm not sure how many of them can be fixed without changing the source content itself, or how to fix most of them, but I think this is a worthwhile start, as it is still readable (an e-book with bad layout is still better than no e-book).

Also added a quick cover image, I'm not a designer, but it gets the job done. Maybe another contributor, with better design skills, can improve it.

[phil-opp]

Thanks a lot for submitting! Unfortunately I'm quite busy right now and I haven't had time to review this PR and to try it out. I try my best to get to it as soon as possible!

[FineFindus]

You're welcome. That's absolutey no problem for me, take your time. In the meantime, I've had time to read through it, using the generated e-book, and noticed a few more issues. Most of them were probably caused by my device, as they do not appear when opening the e-book on my pc, but I think it is still worth mentioning them:

• Code was not line-wrapped, so it sometimes went off-page
• Text was missing in images, which made diagrams unreadable