Standardize documentation in package.R

[nanxstats]

This PR fixes #103.

• Use usethis::use_package_doc() to generate standard package doc page.
• Move some content of the textual blocks in R/package.R to two new vignettes.
• Let the exported functions spendingFunction() and summary.spendfn() have their own proper Rd documentation page and improve documentation style following the tidyverse style guide.
• Link the vignette(s) in the pkgdown function reference page section description like in extending ggplot2.
• Update all Rd @seealso links pointing to the original Rd pages to pointing to the new vignettes.

[keaven]

Possibly add gsBoundSummary(x) in gsDesignPackageOverview.Rmd