New approach to register tidyverse S3 class

[olivroy]

Use new way to register S3 methods with usethis::use_standalone("r-lib/rlang", "s3-register"). https://r-pkgs.org/dependencies-in-practice.html#imports-and-exports-related-to-s3

I also removed some S3 methods from docs, for which it is not very necessary. format(), is.na(), or print methods as they are obvious.

As you see in the preview, the man page looks cleaner! (no more .sf suffix)