Create an app configuration guide

[meryllblanchet]

package.json, manifest.yml, .aio, .env, eslintrc.json, .babelrc, jest.config.js, .vscode, .github (and in the future possible other CI/CD or CDN configuration support...).

We do have a lot of configuration files for a bootstrapped. While we're working on simplifying this (see adobe/aio-cli-plugin-app#395), we should still provide a consistent guide that helps developer understand how to configure their application for development and deployment purposes.

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/AIOE-520