doc: Add guide for building the docs locally

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>

docs/getting_started/docs-build.rst

+Building Documentation
+======================
+
+To create a rendered copy of this documentation locally you can use the
+`Sphinx`_ tool to build and package the plain-text documents into HTML-formatted
+pages.
+
+If you are building the documentation for the first time then you will need to
+check that you have the required software packages, as described in the
+*Prerequisites* section that follows.
+
+.. note::
+   An online copy of the documentation is available at
+   https://www.trustedfirmware.org/docs/tf-a, if you want to view a rendered
+   copy without doing a local build.
+
+Prerequisites
+-------------
+
+For building a local copy of the |TF-A| documentation you will need, at minimum:
+
+- Python 3 (3.5 or later)
+- PlantUML (1.2017.15 or later)
+
+You must also install the Python modules that are specified in the
+``requirements.txt`` file in the root of the ``docs`` directory. These modules
+can be installed using ``pip3`` (the Python Package Installer). Passing this
+requirements file as an argument to ``pip3`` automatically installs the specific
+module versions required by |TF-A|.
+
+An example set of installation commands for Ubuntu 18.04 LTS follows, assuming
+that the working directory is ``docs``:
+
+.. code:: shell
+
+    sudo apt install python3 python3-pip plantuml
+    pip3 install [--user] -r requirements.txt
+
+.. note::
+   Several other modules will be installed as dependencies. Please review
+   the list to ensure that there will be no conflicts with other modules already
+   installed in your environment.
+
+Passing the optional ``--user`` argument to ``pip3`` will install the Python
+packages only for the current user. Omitting this argument will attempt to
+install the packages globally and this will likely require the command to be run
+as root or using ``sudo``.
+
+.. note::
+   More advanced usage instructions for *pip* are beyond the scope of this
+   document but you can refer to the `pip homepage`_ for detailed guides.
+
+Building rendered documentation
+-------------------------------
+
+From the ``docs`` directory of the project, run the following commands. It is
+important to note that you will not get the correct result if the commands are
+run from the project root directory, as that would invoke the top-level Makefile
+for |TF-A| itself.
+
+.. code:: shell
+
+   make clean
+   make html
+
+Output from the build process will be placed in:
+
+::
+
+   <tf-a root>/docs/build/html/
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*
+
+.. _Sphinx: http://www.sphinx-doc.org/en/master/
+.. _pip homepage: https://pip.pypa.io/en/stable/

docs/getting_started/index.rst

@@ -7,6 +7,7 @@ Getting Started
    :numbered:
 
    user-guide
+   docs-build
    image-terminology
    porting-guide
    psci-lib-integration-guide