[doc] Split up the support matrix from the intro.

[trivialfis]

• Keep the intro simple.
• Update references to model IO.

[Copilot]

Pull Request Overview

This PR splits the support matrix from the introduction documentation to improve documentation structure. The main goal is to keep the intro simple while providing a dedicated section for data input type support information.

• Moved the comprehensive support matrix to a dedicated data input documentation file
• Updated documentation references to point to the new location
• Minor text improvements and corrections in documentation and comments

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
python-package/xgboost/core.py	Added polars to supported libraries list and fixed minor typos
doc/python/python_intro.rst	Removed support matrix, updated references, and simplified examples
doc/python/index.rst	Added reference to new data_input documentation
doc/python/data_input.rst	New file containing the complete support matrix moved from intro

Comments suppressed due to low confidence (1)

python-package/xgboost/core.py:956

• The library name 'polars' should be capitalized as 'Polars' to match the proper naming convention used for other libraries in the list (Pandas, Modin, cuDF).

            as Pandas, Modin, polars, and cuDF), The DMatrix recognizes categorical