doc: Split the User Guide into multiple files

The User Guide document has grown organically over time and
now covers a wide range of topics, making it difficult to
skim read and extract information from. Currently, it covers
these topics and maybe a couple more:

- Requirements (hardware, tools, libs)
- Checking out the repo
- Basic build instructions
- A comprehensive list of build flags
- FIP packaging
- Building specifically for Juno
- Firmware update images
- EL3 payloads
- Preloaded BL33 boot flow
- Running on FVPs
- Running on Juno

I have separated these out into a few groups that become new
documents. Broadly speaking, build instructions for the tools,
for TF-A generally, and for specific scenarios are separated.
Content relating to specific platforms (Juno and the FVPs are
Arm-specific platforms, essentially) has been moved into the
documentation that is specific to those platforms, under
docs/plat/arm.

Change-Id: Ica87c52d8cd4f577332be0b0738998ea3ba3bbec
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/design/index.rst b/docs/design/index.rst
index a51a4eb..e3b8f74 100644
--- a/docs/design/index.rst
+++ b/docs/design/index.rst
@@ -6,6 +6,7 @@
    :caption: Contents
    :numbered:
 
+   alt-boot-flows
    auth-framework
    cpu-specific-build-macros
    firmware-design
@@ -13,3 +14,8 @@
    psci-pd-tree
    reset-design
    trusted-board-boot
+   trusted-board-boot-build
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*