DOC: Add inline docstrings to functions in series.py

[Lonercode]

• Removed use of @doc for diff, to_markdown, compare, aggregate and transform methods in /core/series.py.
• Inlined full docstrings directly in the methods mentioned.
• Docstrings are correct.
• This addresses issue DOC: Replace @Appender, @Substitution, @doc with inlined docstrings #62437
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

Thanks @Lonercode