Doc: Improve security documentation of URI/URL/resource

There are security issues with handling URIs and URLs from remote sources (network) and local sources (images, media, and so on).

Go over what we currently cover and see how we can improve the security aspect of those Qt features. Specifically, URL and URI schemes and file loading and saving.

References:

1)There are several CWE entries for input handling, one of them is: 

https://cwe.mitre.org/data/definitions/939.html

2)RFC 8252 has a security consideration section that mentions URI schemes:

https://datatracker.ietf.org/doc/html/rfc8252#section-8