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/pages/rss.xml.ts

@@ -1,5 +1,6 @@
 import rss from "@astrojs/rss";
 import { getCollection } from "astro:content";
+import { getPath } from "@/utils/getPath";
 import getSortedPosts from "@/utils/getSortedPosts";
 import { SITE } from "@/config";
 
@@ -10,8 +11,8 @@ export async function GET() {
     title: SITE.title,
     description: SITE.desc,
     site: SITE.website,
-    items: sortedPosts.map(({ data, id }) => ({
-      link: `posts/${id}/`,
+    items: sortedPosts.map(({ data, id, filePath }) => ({
+      link: getPath(id, filePath),
       title: data.title,
       description: data.description,
       pubDate: new Date(data.modDatetime ?? data.pubDatetime),