refactor: update project data type

- add order property to set explicit ordering on projects page
- make project link optional as not all projects have an accessible link

src/content.config.ts

@@ -23,7 +23,8 @@ const projects = defineCollection({
       description: z.string(),
       tags: z.array(z.string()),
       image: image(),
-      link: z.string().url(),
+      link: z.string().url().optional(),
+      order: z.number().optional(),
       startDate: z.coerce.date().optional(),
       endDate: z.coerce.date().optional(),
     }),