feat: add `partition_rtf` for rich text files (#466)
* refactor epub; add rtf
* added test for rtf files
* filetype detection for rtf files
* add rtf to auto
* update docs for group_broken_paragraphs
* add rtf to docs
* update file list in readme
* update stage_for_transformers docs
* changelog and version bump
* skip rtf if in docker
* skip test if rtf not supported
* docs tweaks
