Improve security documentation around templates

[UlisesGascon]

Based on CVE-2021-32822 and GHSL-2021-020: File disclosure in hbs - CVE-2021-32822 we might want to include recommendations for safe templating in https://expressjs.com/en/advanced/best-practice-security.html.

Passing template engine configuration parameters through the Express render API can lead to vulnerabilities if the object is user controlled. Downstream applications often opt to pass their template data in directly through the remote user-controlled req.query object. This results in a scenario where a remote attacker may be able to subvert the vulnerable application through malicious template engine configuration options.

The security impact is specific to the engine used by the application but ranges from XSS to RCE.