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/utils/getPath.ts

@@ -0,0 +1,36 @@
+import { BLOG_PATH } from "@/content.config";
+import { slugifyStr } from "./slugify";
+
+/**
+ * Get full path of a blog post
+ * @param id - id of the blog post (aka slug)
+ * @param filePath - the blog post full file location
+ * @param includeBase - whether to include `/posts` in return value
+ * @returns blog post path
+ */
+export function getPath(
+  id: string,
+  filePath: string | undefined,
+  includeBase = true
+) {
+  const pathSegments = filePath
+    ?.replace(BLOG_PATH, "")
+    .split("/")
+    .filter(path => path !== "") // remove empty string in the segments ["", "other-path"] <- empty string will be removed
+    .filter(path => !path.startsWith("_")) // exclude directories start with underscore "_"
+    .slice(0, -1) // remove the last segment_ file name_ since it's unnecessary
+    .map(segment => slugifyStr(segment)); // slugify each segment path
+
+  const basePath = includeBase ? "/posts" : "";
+
+  // Making sure `id` does not contain the directory
+  const blogId = id.split("/");
+  const slug = blogId.length > 0 ? blogId.slice(-1) : blogId;
+
+  // If not inside the sub-dir, simply return the file path
+  if (!pathSegments || pathSegments.length < 1) {
+    return [basePath, slug].join("/");
+  }
+
+  return [basePath, ...pathSegments, slug].join("/");
+}