DOC: Documentation improvements

[phofl]

• Add an example of use_nullable_dtypes=True to IO CSV user guide (https://pandas.pydata.org/docs/dev/user_guide/io.html#specifying-column-data-types)
• Add an example of use_nullable_dtypes=True for IO Excel (https://pandas.pydata.org/docs/dev/user_guide/io.html#dtype-specifications)
• Add an example of use_nullable_dtypes=True for IO XML
• Add an example of use_nullable_dtypes=True for IO HTML
• Add an example of use_nullable_dtypes=True for IO SQL
• Add an example of use_nullable_dtypes=True for to_numeric
• Add an example of use_nullable_dtypes=True for IO read_clipboard (see #49995 as example for all of these)
• Provide examples of using read_parquet: DOC: Provide examples of using read_parquet pandas-dev/pandas#49739
• Replace all occurrences of external functions with correct links to their respective documentation (see https://github.com/pandas-dev/pandas/pull/49709/files for a pyarrow example) -> requires finding the occurrences yourselves.
• Document the default encoding for read_csv (utf-8): DOC: read_csv default encoding is not documented pandas-dev/pandas#49881
• Update documentation for str.cat: DOC: .str.cat output in case of Index object pandas-dev/pandas#35556. There was a PR, but it was closed: DOC: Updated docstring for Series.str.cat (pandas-dev#35556) pandas-dev/pandas#35784
• DOC: Fix RT02 issues in docstrings  pandas-dev/pandas#49968

[marielledado]

Working on the read_csv issue pandas-dev/pandas#49881