docs(README): use pak::pkg_install instead of remotes::install_github

[AlbanSagouis]

Summary

• Replace remotes::install_github(...) with pak::pkg_install("fkeck/subtools@dev") in the README installation block. pak is now the recommended tool and handles vignette/manual building automatically.

Notes

• README.md was regenerated from README.Rmd. The whitespace diff in tibble output lines is incidental drift from a newer pillar/tibble version on my system. Happy to revert those if preferred.

Test plan

• README.Rmd edited and re-rendered locally with rmarkdown::render()