doc: Enable automatic labels for page titles

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>