Cross-language comparisons for dataframe manipulation

In the [dataframes](https://exercism.org/tracks/r/concepts/dataframes) concept, there was an original intention to include comparisons between the R/dplyr content and Python/pandas and SQL. In the rush to get the syllabus launched, this aspect was deleted from the `about.md`.

After playing about in the last couple of days, here is a not-totally-serious suggestion for what a more thorough comparison would look like.

| | R | Python | Julia |
|------------|:---:|:------:|:------:|
| data.frame | ✔️ | | |
| tibble | ✔️ | | |
| pandas | | ✔️ | |
| DataFrames | | | ✔️ |
| Polars | ✔️ | ✔️ | ✔️ |
| DuckDB | ✔️ | ✔️ | ✔️ |
| PostgreSQL | ✔️ | ✔️ | ✔️ |
| Arrow | ✔️ | ✔️ | ✔️ |
| Dask | | ✔️ | |

Some of this was on my "should learn in 2026" list back around New Year, so I'll play with it on my GH pages. I doubt much of it will reach the Exercism website! (though maybe some of it should).

Start with the dataframes concept page and the about.md file mentioned in the issue. Compare the existing R/dplyr material with the proposed cross-language table, then confirm the intended scope before editing. Done means the agreed comparison is restored or added to the concept documentation without expanding beyond the settled content.

Written by the indexing model from the issue text.