Unify JS trace sampling docs

[mydea]

Description

Today, we have docs about using tracesSampler as well as tracesSampleRate in the following places:

• https://docs.sentry.io/platforms/javascript/guides/react/configuration/options/#tracesSampler
• https://docs.sentry.io/platforms/javascript/guides/react/configuration/sampling/#setting-a-uniform-sample-rate
• https://docs.sentry.io/platforms/javascript/guides/react/configuration/filtering/#using-traces-sampler
• https://docs.sentry.io/platforms/javascript/guides/react/tracing/configure-sampling/

This seems quite overkill to me. IMHO we should consolidate this to a single place, plus maybe one section with some examples somewhere.