Clarify CSS length inputs in layout docs

[LeonidasZhak]

Closes #1300.

This updates the layout_columns() and layout_column_wrap() documentation to avoid the confusing "CSS length unit" phrasing and gives concrete examples for accepted CSS length values, including numeric pixel values, unit-bearing strings, percentages, and calc() expressions.

Checks run locally:

• Rscript -e "tools::checkRd("man/layout_columns.Rd"); tools::checkRd("man/layout_column_wrap.Rd")"
• git diff --check
• R CMD build --no-build-vignettes .

[CLAassistant]

All committers have signed the CLA.

[LeonidasZhak]

CLA signed, thanks

[LeonidasZhak]

Updated this PR onto the current main, preserving the new offcanvas NEWS entry alongside the documentation note. Validation: both Rd files pass tools::checkRd(), R CMD build --no-build-vignettes succeeds, and R CMD check completes with Status: OK when unavailable Suggests are not forced.

[gadenbuie]

Thanks @LeonidasZhak! I'll take it from here and make sure CI passes before merging 😄