Add documentation

[jeremymiller]

This branch makes all the required edits for a documentation page including:

• Updating the header of functions (e.g., lines starting with #') to be properly formatted with a title, return value, and to omit/fix examples
• Updating the "man" folder (via roxygen::roxygenise()) so help is available when folks type ?[function] in R. This is also a prerequisite for the next step
• Creating a "docs" folder that hosts the documentation website

Note that I did NOT make any changes to documentation (e.g., I used the generic term "value" for all return values and used the function name for all titles). There is plenty of room for future improvement in documentation if that is something you'd like to do.

I did make ONE small edit to an actual function: a correction on line 441 of "harmonize_impute.R" to remove a stray "OA" that was a typo and preventing the library from installing for me.