Add pr_section for fine-grained Word section control in save_flex_docx()

[ddsjoberg]

What changes are proposed in this pull request?

Add a pr_section argument (and matching save_flex_docx-lst:pr_section theme element) to save_flex_docx() accepting an officer::prop_section() object for fine-grained control of the Word section—page margins, page size, orientation, and 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".

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

N/A

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

• Ensured this PR is not the source of a merge conflict
• PR branch is up to date with the base branch (merge or rebase before review)
• Code coverage is suitable for any new functions/features (covr::report())
• usethis::use_spell_check() run and no spelling errors in documentation
• R CMD Check runs without errors, warnings, and notes (relevant test suite passes locally)
• Code is well documented and follows tidyverse style guide

When the branch is ready to be merged:

• 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 pkgdown::build_site() and merge any changes needed

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Notes

This PR is stacked on feat/as_flex_word (PR #2437), not main — it builds on the flextable/list input support from that branch. Please merge #2437 first, after which this PR's base can be retargeted to main (or it will auto-retarget on merge).

Key behavior:

• Header/footer always owned by the function — any header_default/footer_default on the supplied pr_section are discarded in favor of the relocated caption/notes.
• Collections force type = "nextPage" — the same pr_section applies to every table's section; any type on pr_section is ignored between tables to avoid blank pages.
• Always applied — a single table with no caption/notes still gets the section (so margins/size take effect).
• Precedence — the pr_section argument overrides the save_flex_docx-lst:pr_section theme element.

All test-save_flex_docx.R tests pass (NOT_CRAN=true), including 8 new tests covering margins, ours-wins, no-content, collections, list input, theme element + override, and class validation. No NAMESPACE drift; man/ and R/sysdata.rda regenerated.