Document Bootstrap setup

[ajyoon]

Working on some page layouts with this theme, I was extremely confused why my column layouts weren't working as expected. After much debugging and searching I realized the theme sets $grid-columns: 16, instead of the default 12. It would be very helpful if settings like this were clearly documented on the website.

[ajyoon]

On a related note, while looking into this I discovered that this theme uses a beta branch of Bootstrap 5 (5.0.0-beta3), while the readme here had me thinking it was Bootstrap 4. It might be worth updating the dependency now that Bootstrap 5.0 is out, or perhaps pulling bootstrap in as a submodule instead.