Add blog with Nextjs app dir guide

[nkshah2]

Related Issue

Link to the Github Issue created for this blog post

Link to Google Doc

TODO

Checklist

• [ ] Has cover image been added
• [ ] Have all content images been added. Do they render correctly? (aspect ratio etc)
• [ ] The code inside code blocks gives no errors
• [ ] Check for SEO keyword?
• [ ] Added call to action to link to supertokens and to link to other blogs.
• [ ] Add reference to how SuperTokens solves this blog's problem (if relevant).

Remaining TODOs

• [ ] SEO
• [ ] Cover image
• [ ] Table of contents

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