DOC: Add documentation for pandas.api.typing.Expression (#63084)

[pranavkp71]

• closes DOC: missing docs for pd.col Expression objects #63084
• Adds a new documentation page for pandas.api.typing.Expression.

This PR adds missing documentation for the Expression class used by pd.col().
The page includes:

• A short description of the class
• Example usage inside DataFrame.assign
• A list of supported operations (arithmetic, comparison, ufuncs, accessors, and Series methods)

Also linked this page in the API reference index so it appears in the built documentation.