Improve documentation of Encodings extension

The [encodings extension](https://duckdb.org/docs/stable/core_extensions/encodings.html) should list the supported encodings in a table, as the provided Github link is not very usable - it breaks my browser and doesn't show all encodings.

We should also highlight in the examples some of the most common encodings:

* shift_jis (already there)
* windows-1252-2000
* windows-1251-2000
* EUC_CN

It should also be linked in the [CSV reader docs](https://duckdb.org/docs/stable/data/csv/overview.html#:~:text=DuckDB's%20CSV%20reader%20supports%20UTF,the%20iconv%20command%2Dline%20tool.). Currently this recommends using `iconv`, and states that `Options are utf-8, utf-16, latin-1.` - but we now support many more encodings natively through the encodings extension (likely all relevant encodings).

Start with the encodings extension page and the CSV reader overview linked in the issue. Replace the unusable GitHub reference with a supported-encodings table, highlight the requested common encodings in the examples, and link the extension from the CSV documentation with corrected guidance. Done means both pages accurately describe native encoding support.

Written by the indexing model from the issue text.