[DOC] Add Examples sections to validation utility functions

[nimra06]

Describe the issue linked to the documentation

I found several utility functions in aeon/utils/validation/series.py that are missing Examples sections in their docstrings. These functions would benefit from Examples to help users understand how to use them.

Functions missing Examples:

• is_series() (line 18)
• get_n_timepoints() (line 40)
• get_n_channels() (line 86)

These are public utility functions that users might need to understand how to use.

Suggest a potential alternative/fix

Add Examples sections following the numpydoc format used elsewhere in the file (e.g., has_missing() function which has a good example at line 150).

The Examples should demonstrate:

• How to call the function
• What inputs are expected
• What outputs to expect
• Common use cases

[nimra06]

@aeon-actions-bot assign @nimra06