Update the Bootstrap documentation to specifiy the required theme colors

Created by: B-Prod

As explained in the issue #23313 (closed) (see this comment), there are 3 required colors in the $theme-colors variables.

Without one of them, the SASS compilation will fail.

Below are the three color keys that need to be specified:

• primary,
• success
• danger

This is not documented anywhere (or I missed it), so it should be added in the documentation to avoid unnecessary issue reporting or lost of time for developers.