docs(r): fix some pkgdown website linking issues

[eitsupi]

• Fix base URL of each sites. Due to the base URLs, the site search does not appear to be working.
  For example, I searched "adbc" in https://arrow.apache.org/adbc/current/r/adbcdrivermanager/index.html returns https://arrow.apache.org/reference/adbc_driver_log.html?q=adbc#ref-usage
  (On a related note, the search feature seems to stop working when a new version is released and the base URL changes; we can confirm that the search does not work in previous versions of the arrow package.)
• It seems that pkgdown (downlit) uses the first value in the URL field of the DESCRIPTION file to autolinking, and bringing the pkgdown site first seems to be the expected use. (e.g. fix: URL ordering in DESCRIPTION tidyverse/glue#300)

[paleolimbot]

Thank you!