feat: ✨ add 404 page to template (#141)

# Description

This PR adds a 404 page to the template.


This PR needs a quick review.

## Checklist

- [x] Added or updated tests
- [x] Updated documentation
- [x] Ran `just run-all`

Author: martonvago

template/404.qmd.jinja

@@ -0,0 +1,19 @@
+---
+{% if is_seedcase_project -%}
+title: "You've entered the garden of missing pages"
+{% else -%}
+title: "The page you are looking for does not exist"
+{% endif -%}
+---
+
+{% if is_seedcase_project -%}
+Let's get you back to greener grounds.
+
+👉 Go to [homepage](/index.qmd).
+
+![](/_extensions/seedcase-project/seedcase-theme/images/404.svg){fig-alt="An illustration of the number 404 surrounded by trees and mountains"}
+
+## Illustration by [Storyset](https://storyset.com/web) {.appendix}
+{%- else -%}
+This page does not exist, go back to the [homepage](/index.qmd) or check the URL for typos.
+{%- endif %}

template/_quarto.yml.jinja

@@ -5,6 +5,8 @@ project:
   render:
     - "docs/*"
     - "index.qmd"
+    - "404.qmd"
+    - "CONTRIBUTING.md"
 
 website:
   # TODO: Fill in the title of the website.