DOC: clarify Series alignment when assigning to DataFrame column (GH#39845)

[SKTT1Bert]

• Closes DOC: Reindexing behaviour of dataframe column-assignment missing #39845
• Updates the user guide (indexing.rst) to document Series index alignment behavior when assigning to a DataFrame column.
• Adds minimal examples showing:
  • Alignment occurs by index labels, not order.
  • Missing labels in DataFrame index are filled with NaN.
• Built docs locally with Sphinx (-W) without new warnings.
• No code/API changes; documentation only.