Small documentation fixes

[phofl]

• 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.
• 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
• Add an example to https://pandas.pydata.org/docs/reference/api/pandas.read_parquet.html, DOC: Provide examples of using read_parquet pandas-dev/pandas#49739
• DOC: Example missing in pandas.DataFrame.to_html pandas-dev/pandas#44945
• DOC: s3fs is required when using read_csv with an S3 URI pandas-dev/pandas#35206
• DOC: description of record_prefix param for json_normalize is wrong pandas-dev/pandas#54121
• https://github.com/noatamir/scipy-2023-sprint/issues/9
• Specify others in https://pandas.pydata.org/docs/reference/api/pandas.Series.str.cat.html as optional