🚀 Enhancement: Add best practices regarding security in docs

[DH-555]

🔖 Enhancement description

In the security section of the docs, It would be great adding a recommendation to change the default MariaDB and Redis passwords and the _APP_EXECUTOR_SECRET env var for enhanced security.

🎤 Pitch

Those modifications will make databases and executor better secured and improve the security against some unwanted access.

👀 Have you spent some time to check if this issue has been raised before?

• I checked and didn't find similar issue

🏢 Have you read the Code of Conduct?

• I have read the Code of Conduct