Include usage of `ns start` command in `CLI Basics` page

[achou11]

In addition to what's already there, think it would be nice to include some words about how to use the ns start command for getting started (or as the entry point for the other common commands you would use). It was introduced in NativeScript CLI v8.6.0.

To be fair, it's not even documented as a command in the CLI repo itself - which maybe suggests that it's not necessarily ready for official inclusion yet?