feat: Update Next.js snippets and docs for App directory

[anku255]

Summary of change

This PR updates the Next.js snippets and docs for app directory throughout the docs.

Checklist

• [ ] Algolia search needs to be updated? (If there is a new sub docs project, then yes)
• [ ] Sitemap needs to be updated? (If there is a new sub docs project, then yes)
• [ ] Checked for broken links? (Run cd v2 && MODE=production npx docusaurus build)
• [ ] Changes required to the demo apps corresponding to the docs?

Remaining TODOs for this PR

• [ ] Update create-supertokens-app boilerplate for Next.js app dir

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[anku255]

@rishabhpoddar Not sure why the deploy preview failed. I don't have permissions to see the logs.