Add save_flex_docx() to save gtsummary tables and flextables to Word

[ddsjoberg]

What changes are proposed in this pull request?

Added save_flex_docx() to save a gtsummary table or a flextable to a Word (.docx) file via flextable. The header and footer arguments (both default FALSE) optionally place the table caption in the Word document's page header and the footnotes, source notes, and abbreviations in the page footer; when FALSE, that content is rendered as part of the table. The page and page_location arguments add a page-number line (e.g. "Page {PAGE} of {NUMPAGES}") to a chosen header/footer region and alignment. A collection of tables is also accepted—a tbl_split object (from tbl_split_by_rows()/tbl_split_by_columns()) or a plain list of flextables—writing each table to its own Word section and page. The pr_section argument accepts an officer::prop_section() for fine-grained control of the Word section (page margins, size, orientation, columns).

If there is an GitHub issue associated with this pull request, please provide link.

None.

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Summary of changes

• New export save_flex_docx() — writes a gtsummary table or a flextable to a Word .docx file via flextable.
• Accepted input — a gtsummary table, a tbl_split object, a flextable, or a plain list of flextables.
• header / footer arguments (default FALSE) — optionally place the table caption in the Word page header and the footnotes, source notes, and abbreviations in the page footer. When FALSE (the default), that content is rendered as part of the table. When relocated, in-cell footnote reference symbols are retained and only the footnote text is moved. For a flextable, the caption (flextable::set_caption()) is relocated to the Word header and the footer part (flextable::add_footer_lines()) to the Word footer.
• page / page_location arguments — add a page-number line (e.g. "Page {PAGE} of {NUMPAGES}") to a chosen header/footer region and alignment.
• Collection support — a tbl_split object or a plain list of flextables writes each table to its own Word section and page.
• pr_section argument (and save_flex_docx-lst:pr_section theme element) — accepts an officer::prop_section() object for fine-grained control of the Word section (page margins, page size, orientation, columns), while the header/footer regions remain managed by save_flex_docx(); for a collection the same section is applied to every table with the paging type fixed to "nextPage".
• Font matching / inheritance — when relocated, the caption, footnotes, source notes, abbreviations, and page-number line in the Word header/footer match the table body font (family and size) instead of the Word template default; each Word region also inherits the styling applied to the corresponding flextable part (e.g. flextable::fontsize(size = 6, part = "footer") yields a size-6 Word footer).
• Documented limitation — the function help notes that relocated header/footer content is rendered as plain text (markdown/HTML not interpreted, emphasis stripped; only font family/size carried over).
• Tests added in tests/testthat/test-save_flex_docx.R, including the tbl_split/list, pr_section, and page-line paths (skips when flextable/officer are unavailable).
• NEWS.md bullet added; NAMESPACE, man/save_flex_docx.Rd, and the pkgdown reference index regenerated/updated.

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Reviewer Checklist (if item does not apply, mark is as complete)

• PR branch has pulled the most recent updates from main branch.
• If a bug was fixed, a unit test was added.
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• Code coverage is suitable for any new functions/features: devtools::test_coverage()
• usethis::use_spell_check() runs with no spelling errors in documentation
• All GitHub Action workflows pass with a ✅

When the branch is ready to be merged into master:

• Update NEWS.md with the changes from this pull request under the heading "# gtsummary (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
• Increment the version number using usethis::use_version(which = "dev")
• Run usethis::use_spell_check() again
• Approve Pull Request
• Merge the PR. Please use "Squash and merge".