Closes #1760 Updating dropdown menu for articles in website

[zdz2101]

Overview

• Preexisitng fix-1760 branch had recommended code snippet in #1760, modified to ensure articles/index.html presents itself correctly
• Subheaders now in dropdown menu, needed to put text inside of navbar instead of "~"
• Article KRI Method needed to be renamed to achieve this

Test Notes/Sample Code

Notes:

[zdz2101]

@lauramaxwell you mentioned something about some small css modifications for the yeti theme, do you know where to usually make this modification?

[lauramaxwell]

I think you can download the yeti css style file here, edit it and save it in pkgdown/extra.css in the package root. then add a reference to the stylesheet in the header of the _pkgdown.yml. alternatively, we can use a theme like lumen that has similar colors, but better dropdown formatting

[lauramaxwell]

@zdz2101 i'll take a look at customizing the css for the dropdown this afternoon.

[lauramaxwell]

I added some css to make the dropdown headers more legible. Let me know what you think!

[zdz2101]

I think it looks great! I'll have to keep this in mind as a neat little trick