Updated the babashka book section on child process and rewrote it to use babashka.process: https://book.babashka.org/#child_processes

Cross platform single file with deps Clojure JVM launcher script: https://github.com/babashka/book/issues/8

I cleaned up the main README.md as it was getting pretty lengthy and moved most of the content to https://book.babashka.org/ https://github.com/borkdude/babashka#babashka-book

If you think anything is sorely lacking from the README, I can add explicit links to the relevant part in the book. Let me know what you think.