Improve readme code example

[nanxstats]

This PR improves the code example in README.Rmd:

• Update code so it works with under the latest API (fixed_design() -> fix_design_ahr()).
• Remove unnecessary library() calls and only keep gsDesign2. Qualify the other namespaces explicitly.
• Add code chunks for table rendering purposes and remove gt::as_raw_html() from the code chunk for display to reduce confusion and visual clutter.
• Render and refresh README.md so the output reflects the latest table output format.

Also, importantly:

• Use the native pipe |> (as in the purrr readme) to avoid the magrittr dependency where appropriate and possible. This means readme can use |>, but not roxygen2 code examples, which would trigger Depends: R >= 4.1.0 checks.
• In the relevant code chunks, eval is set to getRversion() >= "4.1" so the code chunks will only run when |> exists.

[LittleBeannie]

Thanks for the edits!