fix: fixes unit_size not working in Axes (#2336) and Surprising axis.__init__ default values (#2479)

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Overview: What does this pull request change?

Changes the .__init__() method for axis in the following manner:

• Initially, x_length and y_length are set to None (previously, they were set to round(config.frame_width) - 2)
• If any values for unit_size are passed in x_axis_config or y_axis_config, the necessary length value uses that value.
• Otherwise, the length is set to the default value of round(config.frame_width) - 2.

Motivation and Explanation: Why and how do your changes improve the library?

Links to added or changed documentation pages

https://manimce--4647.org.readthedocs.build/en/4647

Further Information and Comments

This is my first contribution, so please feel free to give feedback!
Have a good day :)

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested