doc: getting_started: Add wget as a common host dependency

This commit adds the instruction for installing the `wget` application
for macOS and Windows hosts, such that it is available on all three
host platforms when the Getting Started Guide is followed.

The rationale behind this is as follows:

  * The Zephyr documentations, including the Getting Started Guide,
    makes extensive use of the wget command.

  * wget is a purpose-made tool for get/download requests and is more
    user-friendly than curl for the purpose of downloading files.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>