docs: ✏️ fix broken link to functions.qmd (#1043)

## Description

The file that is referred to has been renamed.

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This PR needs no review.

## Checklist

- [X] Updated documentation
- [X] Ran `just run-all`

Author: signekb

docs/design/interface/index.qmd

@@ -8,5 +8,5 @@ documents:
 
 -   The main [Outputs](outputs.qmd), including the files, folders, and
     other artifacts that will be produced by Sprout.
--   The [Python functions](python-functions.qmd) that form the core of
+-   The [Python functions](functions.qmd) that form the core of
     Sprout and its extensions.