feat: allow blog posts to be organized by subdirectories

Blog posts can now be structured into subdirectories, which determine their slugs.

Example:
- `/src/data/blog/2025/testing.md` → `/posts/2025/testing`

Directories prefixed with `_` will be excluded from the slug.

Example:
- `/src/data/blog/_examples/tailwind-typography.md` → `/posts/tailwind-typography`

Closes #470, #366

src/content.config.ts

@@ -2,8 +2,10 @@ import { defineCollection, z } from "astro:content";
 import { glob } from "astro/loaders";
 import { SITE } from "@/config";
 
+export const BLOG_PATH = "src/data/blog";
+
 const blog = defineCollection({
-  loader: glob({ pattern: "**/[^_]*.md", base: "./src/data/blog" }),
+  loader: glob({ pattern: "**/[^_]*.md", base: `./${BLOG_PATH}` }),
   schema: ({ image }) =>
     z.object({
       author: z.string().default(SITE.author),