chore: 🚚 simplify name of file to just 'functions' (#1022)

## Description

Originally the name was to indicate the core functions. But moving to
everything being core, I thought it makes more sense to keep this file
simpler in name.

There was also a left over link to a non-existent file in the Quarto
yaml file.

<!-- Select quick/in-depth as necessary -->
This PR needs a quick review.

## Checklist

- [x] Updated documentation

Author: lwjohnst86

_quarto.yml

@@ -47,7 +47,6 @@ website:
         - section: "Architecture"
           href: docs/design/architecture/index.qmd
           contents:
-            - docs/design/architecture/requirements.qmd
             - docs/design/architecture/user-personas.qmd
             - docs/design/architecture/input-data.qmd
             - docs/design/architecture/c4-models.qmd
@@ -56,7 +55,7 @@ website:
           href: docs/design/interface/index.qmd
           contents:
             - docs/design/interface/outputs.qmd
-            - docs/design/interface/python-functions.qmd
+            - docs/design/interface/functions.qmd
     - id: guide
       contents:
         - section: "Guide"

docs/design/interface/python-functions.qmd renamed to docs/design/interface/functions.qmd

@@ -1,5 +1,5 @@
 ---
-title: "Core Python functions"
+title: "Functions and classes"
 callout-icon: false
 callout-appearance: "minimal"
 ---