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

[tsinis]

Description

PR resolves an issue: getsentry/sentry-dart#2912 (review)

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[buenaflor]

Made some modifications if you don't mind :)

@coolguyzone can be reviewed now

[coolguyzone]

Added a suggestion, looks good to me! Thanks for contributing 🏄‍♂️

[tsinis]

Noticed another thing in the docs today - 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?>. If you prefer me to do it in a separate PR, please let me know.