Add user-defined function reference documentation

[Gijsreyn]

PR Summary

This pull request adds reference documentation on how to use user-defined functions.

PR Context

[michaeltlombardi]

Thanks for another great submission, @Gijsreyn!

Thinking on this though, I believe we probably need to get the schema source defined first and then define reference documentation. I've been thinking about the overall structure and probably we need to do something like this:

.
├── docs/
│   ├── concepts/
│   │   └── config/
│   │       └── custom-functions/
│   │           ├── index.md (overview)
│   │           ├── defining.md
│   │           └── using.md
│   └── reference/
│       └── schema/
│           └── config/
│               └── functions/
│                   └── custom/
│                       ├── index.md (overview, usage, array)
│                       ├── definition.md (member object definition)
│                       ├── parameters.md (parameter definition)
│                       └── output.md (output definition)
└── schemas/
    └── src/
        └── config/
            └── document/
                └── document.functions.yaml

And prioritize (maybe across multiple PRs) schema -> reference -> concepts.

This structure would also allow us to break out the definition a bit, which would enable us to define the schema types and so on for every subproperty under an appropriate section heading (instead of nested lists without metadata, like we have here). That's also very helpful for deeplinking to relevant sections.