Add Solution 3 using TrustedHTML to XSS Example

[dschwarz91]

Add a third solution for Example 6 - XSS, add an example of using Trusted Types / TrustedHTML to sanitize the untrusted data

At a first try I couldn't manage to activate it for a single page before the XSS payload got executed, see this comment:

• Add example of XSS vulnerability #24 (comment)