docs: updated manual run with broker examples

[ArtyomVysotskiy]

Description

Fixes #2248 by correcting the "Manual run" example in documentation. The previous example showed incomplete code (app = FastStream(...)) which could confuse users. The new version provides complete examples while maintaining broker neutrality.

Type of change

• Replaced the placeholder code with a complete working examples

Please delete options that are not relevant.

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

Checklist

• My code adheres to the style guidelines of this project (scripts/lint.sh 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
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running scripts/test-cov.sh
• I have ensured that static analysis tests are passing by running scripts/static-analysis.sh
• I have included code examples to illustrate the modifications

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1 out of 2 committers have signed the CLA.

✅ ArtyomVysotskiy
❌ faststream-actions[bot]

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