doc: Enable automatic labels for page titles

Automatic labelling of document titles is a prerequisite for
converting the format of cross-document links. Sphinx will
generate (via the enabled extension) a hidden link target for
each document title and this can be referred to later, from
another page, to link to the target.

The plugin options being used require Sphinx >= 2.0.0 so a
requirements.txt file has been added. This file is used with
the pip package manager for Python so that the correct
dependencies are installed.

Change-Id: Ic2049db5804aa4a6447608ba4299de958ce0a87d
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/requirements.txt b/docs/requirements.txt
new file mode 100644
index 0000000..8f95774
--- /dev/null
+++ b/docs/requirements.txt
@@ -0,0 +1,2 @@
+sphinx>=2.0.0
+sphinx-rtd-theme>=0.4.3
\ No newline at end of file