*Reviewer:* May be quicker to review commit by commit as they are quite
distinct and well-groomed to each focus on a single clean-up task.
Clean up odds-and-ends in the docx partitioner in preparation for adding
nested-tables support in a closely following PR.
1. Remove obsolete TODOs now in GitHub issues, which is probably where
they belong in future anyway.
2. Remove local DOCX "workaround" code that has been implemented
upstream and is now obsolete.
3. "Clean" the docx tests, introducing strict typing, extracting a
fixture or two, and generally tightening things up.
4. Extract docx-local versions of
`unstructured.partition.common.convert_ms_office_table_to_text()` which
will be the base for adding nested-table support. More information on
why this is required in that commit.
Reviewers: I recommend reviewing commit-by-commit or just looking at the
final version of `partition/docx.py` as View File.
This refactor solves a few problems but mostly lays the groundwork to
allow us to refine further aspects such as page-break detection,
list-item detection, and moving python-docx internals upstream to that
library so our work doesn't depend on that domain-knowledge.
