[DOC] Add examples to validation utility functions

[nimra06]

Reference Issues/PRs

No existing issue - found while exploring the codebase.

What does this implement/fix?

Adds Examples sections to utility functions in aeon/utils/validation/series.py that were missing usage examples:

• is_series()
• get_n_timepoints()
• get_n_channels()

The examples follow the existing numpydoc style in the file and demonstrate common usage patterns with NumPy arrays and pandas objects.

Does your contribution introduce a new dependency?

No.

Any other comments?

While exploring the codebase, I noticed these utilities were missing examples, so I added them to improve clarity and consistency with similar functions. This is my first contribution to aeon.

[aeon-actions-bot]

Thank you for contributing to aeon

I have added the following labels to this PR based on the title: [ documentation ].

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[SebastianSchmidl]

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