[TwigComponent] Various fix / cleans in documentation

[smnandre]

This pull request includes several updates to the documentation in the src/TwigComponent/doc/index.rst file. The changes primarily focus on removing versionadded directives, updating component examples, and improving the readability of the documentation.

Documentation updates:

• Removed multiple versionadded directives to streamline the documentation. [1] [2] [3] [4] [5] [6]
• Updated component examples to use simplified syntax and corrected component names. [1] [2] [3] [4]
• Improved explanations and examples for component variants, including compound and default variants. [1] [2]
• Updated a note about the stimulus_controller() function requiring the symfony/stimulus-bundle package.
• Removed outdated references and updated links to external resources.

(PR summary written by)

[Kocal]

Thanks Simon.