docs(dart-dio): add example for dio configuration

[tsinis]

DESCRIBE YOUR PR

Reopens a closed PR due to (false) inactivity. It was already approved once (on May 7, 2025).
PR resolves an issue: getsentry/sentry-dart#2912 (review)

Plus another flaw in the docs - asynchronous Sentry.captureMessage("My message"); call isn't awaited in the current docs, but being suggested to pass into the associatedEventId param, which expects SentryId?, not Future< SentryId?>.

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