Ability to save `gt_group` objects to additional file formats

[bzkrouse]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

Thanks for the new features associated with gt_group! I was able to successfully export multiple {gt} objects into a single file as docx/rtf. It would be great to also have this expanded to additional formats (pdf, html, png, etc.)

[rich-iannone]

Thanks for using these new functions and for the valuable feedback! I can definitely provide more exporting functionality for a group of tables (plus add missing documentation examples).