docs(contrib): add guide to design icons with Affinity Designer (#2262)

* docs(contrib): add guide to design icons with Affinity Designer

* feat(contrib): add Affinity Designer guide to contribution guidelines

CONTRIBUTING.md

···
 
 #### [Figma Guide](https://lucide.dev/docs/figma-guide)
 
+#### [Affinity Designer Guide](https://lucide.dev/guide/design/affinity-designer-guide)
+
 ### Submitting Multiple Icons
 
 If you want submit multiple icons, please separate the icons and group them. That makes reviewing the icons easier and keep the thread clean and scoped.

docs/.vitepress/sidebar.ts

···
           text: 'Designing in Figma',
           link: '/guide/design/figma-guide',
         },
+        {
+          text: 'Designing in Affinity Designer',
+          link: '/guide/design/affinity-designer-guide',
+        },
       ],
     },
   ],

docs/guide/design/affinity-designer-guide.md

···
+---
+title: Affinity Designer Template Guide
+---
+
+# Affinity Designer Template Guide
+
+This guide describes how to use the Affinity Designer template for Lucide.
+
+## General Workflow
+
+>Attention: By default, Affinity Designer sets the unit for stroke to points. Make sure that it is set to pixel. To do this, open `Preferences > User Interface`. Under `Decimal Places for Unit Types`, uncheck `Show Lines in points`.
+
+1. Download and open the [Affinity Designer template](https://github.com/lucide-icons/lucide/blob/main/docs/public/templates/affinity_designer.aftemplate).
+2. Follow the [Icon Design Principles](icon-design-guide.md) while you use the template (to ensure integrity with the Lucide icon pack).
+3. Export the file as SVG (`File > Export`). Make sure that _Rastering_ is set to _Nothing_, _Export text as curves_ is checked (hopefully, you won't need this), _Use hex colors_ is checked, and _Flatten transforms_ is checked.
+   
+	 ![SVG export options in Affinity Designer](../../images/affinity-designer-export-options.png?raw=true)
+4. Optimize the exported SVG file further with [SVGOMG](https://jakearchibald.github.io/svgomg/) or [`svgo`](https://github.com/svg/svgo) (using `svgo --multipass exported_icon.svg`).