feat(perf): Restructure performance getting started

[AbhiPrasad]

High level goals

• Remove the dropdown in https://docs.sentry.io/product/performance/getting-started/ (because users get very confused) in favour of a list of links. Doing this also means that the getting started pages are all unified (error getting started and performance getting started for each SDK).

• Each link in that list should reference performance specific documentation under that SDK platform

• Make sure that all the pages have links at the bottom so that users can easily navigate between different sections

• Make sure performance set up is unified across languages

Roughly:

1. Install SDK
2. Edit init to add tracesSampleRate
3. Auto instrumentation -> link to auto instrumentation framework or specific docs
4. Manual instrumentation
5. Use cases and examples
6. Special options (SDK specific behaviour)

• Explore where performance documentation should live. I think we can experiment and as long as the links are stable (or redirect), we can move stuff around afterwards.

This also came up because if we add Angular to the getting started dropdown, it replaces Browser JavaScript as the first thing rendered, which I don't think is ideal.

WIP:

• Have Vue, React left, also want to add Gatsby tracing docs + unify Angular tracing docs.

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[PeloWriter]

Minor edits. Thank you.