Small documentation improvements

[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: Document the filters argument in read_parquet pandas-dev/pandas#52238
• DOC: describe any expected ordering of value counts when sort=False pandas-dev/pandas#51420 (needs either knowledge about value_counts or testing the described cases)
• DOC: MultiIndex methods not grouped together pandas-dev/pandas#50496
• DOC: missed behavior explaination of sort=False for groupby pandas-dev/pandas#47529
• DOC: converting Index object with duplicate entries into a set pandas-dev/pandas#46440
• DOC: Example missing in pandas.DataFrame.to_html pandas-dev/pandas#44945
• BUG: to_json()/read_json() can't correctly dump/load numbers requiring >15 digits of precision pandas-dev/pandas#38437 (document limitation)
• DOC: s3fs is required when using read_csv with an S3 URI pandas-dev/pandas#35206
• DOC: index_col parameter of pandas.read_excel can be types other than int pandas-dev/pandas#52716

[chennnxu]

take DOC: Example missing in pandas.DataFrame.to_html #44945