doc: develop: Add 'Toolchains' sub-category

This commit adds the 'Toolchains' sub-category under the 'Developing
with Zephyr' category with toolchain-related documentations that were
previously located under the 'Getting Started Guide' sub-category.

It also reworks the toolchain-related documentation in the "Beyond
Getting Started Guide" document such that it is more up to date and any
confusing and/or misleading details are removed.

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