bundle interfaces under a new category 'data sources'

[tonne1]

One of the outstanding features of duckdb is its interfaces to other data sources. So far, data sources supported by duckdb are scattered around the docs under "data import", "guides/file formats" and "extensions". If there was a top-level category "data sources", one could get a quick overview of which sources are currently supported, if natively or through extensions, and perhaps if (more or less) stable or experimental:

• CSV
• Excel
• JSON
• Parquet
• httpfs
• MySQL
• PostgreSQL
• SQLite
• ...

That could also be a good place to document small examples of reading from/writing to each of them. Plus links to the corresponding sections within the docs for details. Each of them should have its options documented too.

'Interfaces' could be another name for 'data sources', but is probably too general.