doc: Migrate content from User Guide and update it

The User Guide document holds many different bits of
information that are not always related to each other. These
are moved out into files where information is grouped tightly
together.

This also updates build instructions and environment variable
instructions to link to the relevent parts of the TF-A user guide
and use more recent versions of compiler, respetively.

Change-Id: Iace851f93c7dea8f154ae52a2665dd599f2e5d72
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
index e9006e7..1436767 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -8,7 +8,7 @@
    Home<self>
    about/index
    getting_started/index
-   user-guide
+   process/index
    porting-guide
    implementing-tests
    design
@@ -65,7 +65,6 @@
 
 *Copyright (c)2019, Arm Limited. All rights reserved.*
 
-.. _FVP: https://developer.arm.com/products/system-design/fixed-virtual-platforms
 .. _Juno Arm Development Platform: https://developer.arm.com/products/system-design/development-boards/juno-development-board
 
 .. _Power State Coordination Interface (PSCI): PSCI_
@@ -85,6 +84,3 @@
 .. _Firmware update: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/about/docs/firmware-update.rst
 .. _EL3 payload: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/about/docs/user-guide.rst#el3-payloads-alternative-boot-flow
 .. _Secure partition: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/about/docs/secure-partition-manager-design.rst
-
-.. _issue tracker: https://developer.trustedfirmware.org/project/board/9/
-.. _mailing list: https://lists.trustedfirmware.org/mailman/listinfo/tf-a-tests