Make `DataFrame` and `LazyFrame` docstring examples follow `Expr` and `Series` templates

[MarcoGorelli]

The DataFrame and LazyFrame docstrings were added a while ago, and they follow a different pattern compared with the more modern Expr and Series ones

I much prefer the newer ones - they allow us to really hammer-home the message that Narwhals is meant as an internal tool with which you can make your functions dataframe-agnostic

Can we update the DataFrame and LazyFrame docstrings examples to follow this template too? The template is:

1. import pandas, Polars, and Narwhals
2. define data = ..., and make df_pd and df_pl dataframes
3. define a dataframe-agnostic function which uses the given method
4. show that you can pass either pandas or Polars to the given method
5. if there are any differences users need to be aware of, add a "Notes" section (e.g. null handling differences)

[mfonekpo]

I will work on this

[FBruzzesi]

Mentioning here hoping to simplify visibility. The methods that are still not following such template are:

• DataFrame
  • [x] .join(..) (WIP in #347 )
  • [ ] .sort(..)
• LazyFrame:
  • [x] .join(..) (WIP in #347 )
  • [ ] .sort(..)
  • [ ] .columns
  • [ ] .collect()
  • [ ] .drop(..)
  • [ ] .rename(..)
  • [ ] .unique(..)