Document all of usethis's options in one place

usethis needs a help topic like these:

* https://gargle.r-lib.org/reference/gargle_options.html
* https://tibble.tidyverse.org/reference/tibble_options.html

(Some of) the options to document there:

* `usethis.destdir`
* `usethis.description`
* `usethis.protocol`
* `usethis.quiet`
* `usethis.overwrite`