docs: Add section or page for special functions

[Omega359]

Is your feature request related to a problem or challenge?

As mentioned in #12948 (comment) there are a few scalar functions that are not normal UDF's and cannot be documented in the standard manner as per #12740 Thus we need to find a way to add documentation for those functions to the user guide once the scalar function documentation is completely migrated over to the new code driven design.

Describe the solution you'd like

A few idea:

• Hardcode a section into the generated scalar-functions.md for these functions
• Add a new page to the documentation that outlines 'special' functions

Describe alternatives you've considered

No response

Additional context

No response

[jonathanc-n]

take

[Omega359]

Here is a list of the remaining 'special' functions as I see it. Unnest is really the only special function, the others are just existing functions with different syntax such as substring('Thomas' from 2 for 3) → hom.

• unnest (array), unnest(struct)
• extract - it's data_part, but with different syntax
• position - it's strpos, but with different syntax
• substring, it's substr, but with different syntax
• trim, it's one of the trim functions, but with different syntax

I am unsure right now how to properly document the 'different syntax' types of functions. Perhaps these should be covered in a page that covers things like ilike, cube, trim, etc.