Document separator groups for `recursive` chunking strategy

[kosabogi]

This is a follow-up PR for #2946.

Currently, the available separator_group values for the recursive chunking strategy are not documented. Users can configure custom separators, but if they want to rely on predefined separator groups, there’s no reference in the docs that explains what these groups contain.

Add documentation for the separator_group options in the chunking settings related documentation.
Add cross-links from related pages, for example at the Inference API chunking settings.

Code reference:
Separator groups are defined in org.elasticsearch.xpack.inference.chunking.SeparatorGroup