feat: add sentry

[scefali]

This PR adds Sentry error, performance, and session replay monitoring. Note Sentry offers both a SaaS solution and
self-hosted solution. The instructions you are using SaaS but the guide still works with self-hosted with a few modifications.

Also note that because Next.js requires environment variables during build time for Sentry, we have to pass the environment variables through Docker arguments and modifying the local fly.toml which is in the .gitgnore (otherwise we wouldn't be able to add the sensitive environment variable to it).

Closes #28

[scefali]

@ykhli think you can have someone take a look?

[ykhli]

thank you for the PR @scefali ! Sorry for the delay - i have been traveling this week. Will take a look soon and let you know if there are questions.

Appreciate it! Excited to try this out

[timqian]

Hi, @scefali Thanks for your contribution!
Sentry is a good plugin for people who need monitoring, but for people who want to start quickly, it may add extra complexity for them. So I'm a bit concerned about directly adding it to the template.
We are planning to build a CLI which allows users to choose which plugin they need. (#43) I think adding sentry in that way may be better for users