Update documents in [Data ingestion]

[jhk0530]

Hi, I'm trying duckplyr as an alternative to dplyr.
and following with documents from pkgdown.

and I found some of the documentation could be improved.

in Reference

At now, documents shows like below.

`df_from_csv()` `duckplyr_df_from_csv()` `df_from_parquet()` `duckplyr_df_from_parquet()` `df_to_parquet()` `df_from_file()` `duckplyr_df_from_file()`
df_from_csv

but this may change as below.

`df_from_csv()`, `duckplyr_df_from_csv()`
- dataframes from csv
`df_from_parquet()`, `duckplyr_df_from_parquet()`
- dataframes from parquet
`df_to_parquet()`
- write dataframes to parquet
`df-from_file()`, `duckplyr_df_from_fle()`
- dataframes from json 

in files, I mentioned only json which is not sure, but I found only json in duckdb docs

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I think issue #135 tells there are many functions which is not documented yet, so this may not be prior task. but one of starting point with it.

Also this is another question, (not important) duckplyr supports parquet only as exporting result.
If I wanted to save result as a CSV, could I simply use write.csv()?

Thanks.

[krlmlr]

Thanks, Jinhwan, for raising this. I'll review when I next work on the documentation.