doc: Document DESCRIBE comman in ddl.md

[krikera]

Add documentation for DESCRIBE and DESC commands with syntax, examples, and output format explanation.

Fixes #16518

Which issue does this PR close?

• Closes doc: Document DESCRIBE command in ddl.md #16518.

Rationale for this change

The DESCRIBE command was not documented in the user guide, making it difficult for users to discover and use this feature.

What changes are included in this PR?

Adds documentation to docs/source/user-guide/sql/ddl.md including:

• Syntax for both DESCRIBE and DESC aliases
• Output format explanation (column_name, data_type, is_nullable)
• Three examples: basic usage, DESC alias, and external tables
• Compatibility note for all table types

Are these changes tested?

The DESCRIBE command functionality is already tested in datafusion/sqllogictest/test_files/describe.slt and information_schema.slt. This PR only adds documentation.

Are there any user-facing changes?

Yes - users can now find DESCRIBE command documentation in the official user guide with clear usage examples and expected output format.

[alamb]

Looks great to me -- thank you @krikera

FYI @comphead

[alamb]

I took the liberty of pushing a commit to get the CI to pass

[comphead]

Thanks @krikera for the nice first contribution!