docs: improve doctests for complex number instances in math/base/special/csignumf

[prajjwalbajpai]

Resolves a part of #8641.

Description

This pull request:

• Updates REPL and documentation examples in the maths/base/special/csignumf package to use the compact complex instance doctest notation.
• Replaces example decomposition using realf/imagf with concise doctest annotations.

Related Issues

This pull request has the following related issues:

• [RFC]: Improve doctests for complex number instances in documentation examples (tracking issue) #8641

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• Read, understood, and followed the contributing guidelines.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/csignumf	$\color{green}170/170$ $\color{green}+0.00%$	$\color{green}7/7$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}170/170$ $\color{green}+0.00%$

The above coverage report was generated for the changes in this PR.

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[kgryte]

LGTM apart from the C changes, which were not needed.