Add note to dashboard example to indicate it is NOT mobile friendly

Created by: dhthwy

The example located at https://getbootstrap.com/docs/4.4/examples/dashboard/ loses its navigation bar on small viewports. This causes the dashboard to lose major functionality on these devices. Given that bootstrap purports to be "Mobile First," having examples that are broken for mobile devices causes confusion and time waste to users attempting to learn the bootstrap framework and best practices (which arguably should be found in the examples).

There are multiple tickets in regards to this issue: #26311 (closed) #26884 (closed) A note could inform users that the example isn't suitable for mobile devices and help curb confusion and time wasted for everyone in dealing with this problem.