[RR7]: Add a guide for basic instrumentation

[chargome]

Description

Until we provide dedicated SDK support for React Router v7 (framework), we should create documentation on how to help users instrument their RR7 applications using our existing React and Node SDKs.

This should be considered a temporary solution until a dedicated SDK is available

[ecline123]

So much yes for this! Especially if you're not going to leverage browser tracing, just some simple pointers for how to instrument a RR7 app with the sentry/node and sentry/react packages.