SDK documentation for Performance Monitoring

[bruno-garcia]

In this PR we want to address some of the points of this issue #221

Goals:

• Better general guidelines for SDK developers to implement performance monitoring support
• FAQ section at the end with common questions that come up
• Links to source code of different SDKs to demonstrate how it was done so far
• Clearly communicate some differences (EventProcessor is called in JS but not in others SDKs necessarily).

[vercel]

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✅ Preview: https://develop-git-ref-sdk-performance.sentry.dev

[rhcarvalho]

The redirects don't seem to be working in the preview deployment, but otherwise nothing is broken so feel free to merge and iterate on it later.

@bruno-garcia we could update https://github.com/getsentry/develop/blob/master/src/docs/sdk/index.mdx to make the Perf Mon page part of the "home page" for SDK docs, makes it easier to navigate to it.

[rhcarvalho]

Idea: if one of the problems is this page is hard to find, try adding tags: [...] here in the frontmatter, I heard it is picked up by Algolia search.

[bruno-garcia]

OK merged, but lets follow up on this an iterate