`use_pkgdown_url()` doesn't check if DESCRIPTION already has URL

[dchiu911]

This situation can occur when I am already using a pkgdown + github pages workflow but without CI. My DESCRIPTION URL field already has the github pages link, but after running usethis::use_pkgdown_github_pages() to migrate to github actions, I noticed that use_pkgdown_url() doesn't check for an existing URL before adding to DESCRIPTION.