Add article for building tables using the tbl_ard_() functions

[ddsjoberg]

Use usethis::use_article() to create the article. We'll want to include the following:

• Building a basic demographics table, including how to add an overall column
• A disposition table
• An AE table
• A more complex statistical results table that requires us to "hack" the standard table format. For example, we could include a subgroup analysis utilizing tbl_ard_continuous()?

We'll model the tables after tables in the FDA specs and use these data: https://github.com/pharmaverse/pharmaverseadam