1st Round cleanup/re-org of pkgdown vignettes

[LittleBeannie]

This issue involves the initial round of cleanup and reorganization of the pkgdown vignettes. The main objective is to remove duplicated content and restructure the table of contents on the pkgdown/article page.

For instance, the content of the following vignettes primarily focuses on the implementation of R functions, which can be incorporated as examples within the help files.

• usage-expected-accrual.Rmd -> examples in expected_accrual.R
• usage-expected-time.Rmd -> examples in expected_time.R
• usage-summary-as-gt.Rmd -> examples in summary.R and as_gt.R
• best-practice-npe-accuracy.Rmd -> examples in gs_power_npe.R

With this issue resolved, we will proceed with another round of cleanup for the vignettes, tentatively planned for around 2024 Q2. The focus of this cleanup will be on the content of the vignettes, including tasks such as adding new material, dividing certain vignettes into two, or merging multiple vignettes.