Add support to read docx files

[taabishm2]

Changes: - Implement parse_docx_to_text function to handle .doc and .docx files. - Use text chunking logic for splitting up docx text after parsing

Bug-fixes:
- Convert file names in `paperqa/readers.py` to lowercase before checking extensions to ensure case-insensitive matching.

[taabishm2]

Looking good. Added some comments, and can you make a unit test?

@jamesbraza Good points. Pushed some changes, please take a look

[whitead]

Thanks for your contribution! This is now stale - please open a new PR if you're still interested.