[JS SDK] Update docs around `@sentry/tracing` usage

[AbhiPrasad]

We have removed the need to use @sentry/tracing - let's update the docs accordingly.

### Tasks
- [ ] https://github.com/getsentry/sentry-docs/pull/6508
- [ ] https://github.com/getsentry/sentry-docs/pull/6523
- [ ] https://github.com/getsentry/sentry-docs/pull/6565