Create an app configuration guide

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 https://github.com/adobe/aio-cli-plugin-app/pull/395), we should still provide a consistent guide that helps developer understand how to configure their application for development and deployment purposes.

Review the configuration files named in the issue—package.json, manifest.yml, .aio, .env, eslintrc.json, .babelrc, jest.config.js, .vscode, and .github—alongside the linked simplification pull request. Document how these files support application development and deployment, and consider future CI/CD or CDN configuration support. Done means developers have one consistent configuration guide.

Written by the indexing model from the issue text.