[CIVIS-1869] DOC Add confirmation of compressed file usage in civis_file_to_table

[Drew-Pappas]

Tested and confirmed that .gz files can be used in civis.io.civis_file_to_table and added a section to the docstring.

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed