docs: add "testing" section to getting started

[ArtyomVysotskiy]

Description

Fixes #2524

Added testing examples for each broker in "Getting Started" section

Type of change

• Documentation (typos, code examples, or any documentation updates)

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• Both new and existing unit tests pass successfully on my local environment by running just test-coverage
• I have ensured that static analysis tests are passing by running just static-analysis

[CLAassistant]

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✅ ArtyomVysotskiy
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[ArtyomVysotskiy]

@Lancetnik Hi! Can u check this one?

[Lancetnik]

@ArtyomVysotskiy can you please add these test run to tests/doc directory?

[ArtyomVysotskiy]

Yeah, of course!

[Lancetnik]

@ArtyomVysotskiy sorry, have we any updates here?

[ArtyomVysotskiy]

@Lancetnik Hi, sorry, I've had a couple of projects come up and I just can't find the time :(