docs: clean docs for $list methods

[etiennebacher]

Style:

• remove useless @format and @keywords
• harmonize examples:
  • reuse same data when possible
  • use with_columns() instead of select() to show input and output side-by-side
  • when applicable, have an example with Expr and one with literal
• fix typos and weird sentences
• better titles

Other:

• deprecate $list$lengths(), use $list$len() instead
• remove the S3 [ that was an alias for $list$get(), it just seemed weird to me to have this single method in there and I can't imagine it's easier to use. Ok to revert if necessary though

[eitsupi]

Great!

Could you update NEWS as a breaking change?

[etiennebacher]

Thanks, done