Docs: Add APP_SECRET generation step for the demo application

[malsuke]

When following the current documentation to set up the Symfony Demo application, an error occurs on startup because the APP_SECRET variable is not set in the .env file.

To fix this and allow the server to start, the following command must be run manually:

php bin/console secrets:set APP_SECRET

To resolve this issue, I propose adding the command to set APP_SECRET to the documentation:

symfony new my_project_directory --demo
+ cd my_project_directory
+ php bin/console secrets:set APP_SECRET

How to Reproduce

1. Run the following commands as instructed by the documentation:

symfony new my_project_directory --demo
cd my_project_directory
symfony server:start

2. Open http://localhost:8000 (or the server's address) in a browser.

An error page is displayed, indicating that secret is missing.

[94noni]

Isnt this fixed with recent symfony/demo#1579 ?