doc: west: move advanced content out of 'basics'

With the documentation for west 0.9, the 'basics' page is completely
out of control. Reorganize it so it fits in a few pages, moving many
details out into other pages for the truly curious.

As part of this, create new pages for built-in commands (built-in.rst)
and esoteric details about workspaces (workspaces.rst).

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>