[Docs-Change] Add reference for functions

[Luna-Klatzer]

Describe the change

Add the reference section for functions, which should contain the following items:

• Defining/declaring a function, signature of a function.
• Calling a function and using parameters

These should contain the standard items such as Syntax, Examples and Additional Notes if required

Additional context

Section: https://para-c.readthedocs.io/en/latest/lang_ref/functions/index.html

[Luna-Klatzer]

Marking this as dev-branch-only, as the current docs are in dev and not pushed to main. This will change in the next few days with the release of 0.1.dev5, where docs changes may be pushed to main directly again to be also viewed by others right after being merged.