update fastify doc and quickstart

[victoriaxyz]

Important

🔎 Previews:

• https://clerk.com/docs/pr/1555/quickstarts/fastify

Explanation:

This PR:

• refactors the setup instructions
• updates code examples
• adds important notes about setup

Todo

• [Done] Clerk Dashboard > Configure > Quick Copy dropdown does not have a Fastify option
• [Done] Pending access request to Linear Design team, need to make ticket to get Fastify icon for <TutorialHero/>
• [Done] Repo name needs to be updated to clerk-fastify-quickstart

Notes

• Updated quickstart
• Unsure if previous starter repo worked since there was no tsconfig.json file to run server
• Is it necessary to have a Clerk Fastify doc to show getAuth() method? E.g., Remix
• Is the ESM note about package.json is necessary?

[github-actions]

Hey, here’s your docs preview: https://clerk.com/docs/pr/1555