docs: Add documentation for functions module

[f4t4nt]

This PR adds documentation for the functions module and refactors several functions from expressions to the functions module. Key changes:

• Moves list_, struct, and coalesce from expressions to the dedicated functions module
• Adds new functions.rst file with structured documentation
• Organizes functions into logical categories (sequence, general, constructor)

[f4t4nt]

I didn't move coalesce, list, struct to functions for now as that seems more involved (ie have to move things around on the rust side too?). Maybe that can be another PR?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.78%. Comparing base (567ae9a) to head (860faad).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3880      +/-   ##
==========================================
- Coverage   78.46%   77.78%   -0.69%     
==========================================
  Files         767      769       +2     
  Lines       97108    97838     +730     
==========================================
- Hits        76193    76100      -93     
- Misses      20915    21738     +823     

see 39 files with indirect coverage changes