add api docstring to with_requirements

[victordibia]

Starting out this draft PR to add documentation for the with_requirements decorator in the autogen-core package.

Why are these changes needed?

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
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[codecov]

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All modified and coverable lines are covered by tests ✅

Project coverage is 74.96%. Comparing base (dd0781a) to head (2dc99b7).

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  Coverage   74.96%   74.96%           
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  Files         175      175           
  Lines       11158    11158           
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  Hits         8365     8365           
  Misses       2793     2793           

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[victordibia]

@jackgerrits, as part of #5601, I have added some documentation to with_requirements decorator.
My understanding of its core function is that it is a way to specify packages/imports used by a code executor.

Additionally, we need to ensure this works correctly with component config
Can you explain a bit more what is meant here? e.g., an example of what exact component is being dumped.

Note: In the current implementation of serialization for command line executors, we do not try to serialize/load passed in functions. This is mainly because the type is quite broad.

 def __init__(
        self,
        timeout: int = 60,
        work_dir: Union[Path, str] = Path("."),
        functions: Sequence[. ## <- not serialized or loaded
            Union[
                FunctionWithRequirements[Any, A],
                Callable[..., Any],
                FunctionWithRequirementsStr,
            ]
        ] = [],
        functions_module: str = "functions",
        virtual_env_context: Optional[SimpleNamespace] = None,
    ):