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/plat/marvell/build.rst b/docs/plat/marvell/build.rst
index c8923e4..c10bcff 100644
--- a/docs/plat/marvell/build.rst
+++ b/docs/plat/marvell/build.rst
@@ -194,8 +194,8 @@
     can interrupt UART recovery process). This MACRO definition is set in
     ``plat/marvell/a8k/common/include/platform_def.h`` file.
 
-For more information about build options, please refer to section
-'Summary of build options' in the :ref:`User Guide`.
+For more information about build options, please refer to the
+:ref:`Build Options` document.
 
 
 Build output
@@ -251,4 +251,3 @@
     (use the "mv_ddr-armada-atf-mainline" branch):
 
     https://github.com/MarvellEmbeddedProcessors/mv-ddr-marvell.git
-