doc: gsg: Update index.rst for macOS

Added python-tk to the brew install statement.
In order to use the zephyr gui tools python must also have the
"tkinter" (python tk interface) tools available.

tkinter is NOT part of 'python3' through homebrew on macOS.

fixes #76811

Signed-off-by: Eric Southam <eric.southam@lvt.com>