Revamp the custom user functions documentation

[petar-i-todorov]

Currently, when we point users to a solution that uses custom user functions, we typically provide the following documentation articles for guidance on how to add custom user functions:

• https://docs.telerik.com/reporting/designing-reports/connecting-to-data/expressions/extending-expressions/overview
• https://docs.telerik.com/reporting/designing-reports/connecting-to-data/expressions/extending-expressions/user-functions

However, I believe that the documentation for custom user functions should be more comprehensive and centralized, and effectively address common issues. For instance, there have been ticket cases in the past where users encounter difficulties adding custom user functions locally, even after reviewing the existing articles.

Consider applying the following changes:

• Provide examples of how custom user functions can address real-life situations.
• Clarify the distinction between custom user functions in the report class and those from a separate assembly. Consider emphasizing this difference in the section headings. It might also be helpful to mention these distinctions at the beginning of the article and include an unordered list with items pointing to the relevant sections (an example can be found here).
• Cover some common caveats, such as the difference between the .NET/.NET Framework SRD version, and why it is important to use the correct one.