refactor: consider adding a `welcome` book that is not in the root folder

[moliholy]

Currently there are four gitbooks in the repository:

• welcome (in the root folder)
• pop-cil-for-appchains
• pop-cli-for-smart-contracts
• pop-network

There is no reason to keep the welcome gitbook in the root folder instead of its own. If it'd be in its own folder, we'd be able to place a proper README file that showcases how to use this repository. All of it is currently in the CONTRIBUTING.md file, but that's not ideal.