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/reset-design.rst b/docs/design/reset-design.rst
index a92ceeb..7b10c95 100644
--- a/docs/design/reset-design.rst
+++ b/docs/design/reset-design.rst
@@ -115,8 +115,8 @@
 It allows the Arm FVP port to support the ``RESET_TO_BL31`` configuration, in
 which case the ``bl31.bin`` image must be loaded to its run address in Trusted
 SRAM and all CPU reset vectors be changed from the default ``0x0`` to this run
-address. See the :ref:`User Guide` for details of running the FVP models in this
-way.
+address. See the :ref:`Arm Fixed Virtual Platforms (FVP)` for details of running
+the FVP models in this way.
 
 Although technically it would be possible to program the reset base address with
 the right support in the SCP firmware, this is currently not implemented so the