Create a section `Data Sources` and create under this section separate pages for all supported data sources

[zaleslaw]

up to the first level the pages related to reading from Data Sources: Files, SQL databases, URLs look at the laconic Polars IO https://docs.pola.rs/user-guide/io/

Use the content from these pages: https://kotlin.github.io/dataframe/io.html
Create structure like:

Data Sources

• CSV
• JSON
• Excel
• Arrow
• SQL
  • PostgreSQL
  • MySQL
  • MS SQL
  • SQLite
  • H2
  • MariaDB
  • Custom SQL Source
• Unsupported Data sources (Add a chapter "Integration" for the Guides of unsupported data sources or frameworks which has no special module, like Exposed, Spark, DuckDB and so on #1298)

It should be after Concepts, but before Operations

[Jolanrensen]

I added Unsupported Data sources (#1298) to the end, because I think here is the best place to add it. It's where people will look when they want to see if their favorite data source is supported or not.