Update AppSec Kit configuration documentation

[tamasmak]

Adds the vaadinPlatformVersion configuration option description to the AppSec Kit configuration documentation.

[github-actions]

AI Language Review

In the file Advanced Topics, an additional configuration option, vaadinPlatformVersion, has been introduced. Ensure that the description clearly specifies that Vaadin platform BOM refers to the Bill of Materials for the Vaadin platform, to avoid any confusion. Additionally, clarify the process of determining the Vaadin platform version when vaadin-core dependency is absent, to ensure users understand how the AppSecException can be avoided by defining the version in the configuration option.

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✅ peholmst
❌ heruan
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