docs(build): clarify getting started section

The Getting started section is very difficult to follow. Building the
fip comes before building the files it needs, the BL33 requirement is
given in a somewhat hand wavy way, and the Arm Developer website
download provides a lot of targets and the guide is not clear which ones
are needed on download.

Swapping the initial build and supporting tools sections makes the flow
more natural and the supporting tools section then becomes clear.
Explicitly mentioning the GCC targets avoids confusion for people less
familiar with the project (eg. new starters).

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I02e88f8c279db6d8eda68f634e8473c02b733963
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 62ef826..7eabfa3 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -27,6 +27,10 @@
 target the Armv7-A or Armv8-A architectures:
 
 - GCC >= 11.3.Rel1 (from the `Arm Developer website`_)
+
+   You will need the targets ``arm-none-eabi`` and ``aarch64-none-elf`` for
+   AArch32 and AArch64 builds respectively.
+
 - Clang >= 14.0.0
 - Arm Compiler >= 6.18