feat(docs): Improve the toolchain setup page (#2272)

Split the toolchain setup into separate docker and native pages
and improve instructions to better refer to Zephyr docs in certain steps.
Also refactor to improve consistency and add virtualenv instructions.

---------

Co-authored-by: KemoNine <mcrosson@kemonine.info>
Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

docs/src/css/custom.css

@@ -46,3 +46,7 @@
   width: 100%;
   height: 100%;
 }
+
+.secrettabs {
+  display: none;
+}