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This new page contains instructions for doing a local
build of the documentation, plus information on the environment
setup that needs to be done beforehand.

Change-Id: If563145ab40639cabbe25d0f62759981a33692c6
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

This change creates the following directories under docs/
in order to provide a grouping for the content:

- components
- design
- getting_started
- perf
- process

In each of these directories an index.rst file is created
and this serves as an index / landing page for each of the
groups when the pages are compiled. Proper layout of the
top-level table of contents relies on this directory/index
structure.

Without this patch it is possible to build the documents
correctly with Sphinx but the output looks messy because
there is no overall hierarchy.

Change-Id: I3c9f4443ec98571a56a6edf775f2c8d74d7f429f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>