Renamed the "Basic Setup" page to "Toolchain Setup" to make it clearer
that this is for setting up a development environment for local builds.
Linked to the relevant Zephyr setup documentation instead of duplicating
that information in our guide. This also switches Windows and macOS
setup to using the Zephyr SDK, which is much easier to install.
Updated the command for installing Zephyr's Python dependencies to be
OS specific, as Windows and macOS don't need the --user flag.
Updated the IDE integration page with instructions for determining the
compiler path that work with the latest version of the Zephyr SDK.
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