Dataframe chapter update (#1391)

* mentioning polars plugin installation

* update dataframe examples output

* rewritten installation instructions

* updated examples with numd and default theme

numd run dataframes.md --prepend-intermid "$env.config.footer_mode =
'never';
∙ $env.config.table.header_on_separator = false; $env.config.table.mode
= 'default'; $env.config.table.abbreviated_row_count = 10000"

* remove ```output-numd fences because CI don't like them