doc: Fix encoding problem

Depending on the version of Python 3 the user has installed, 'open'
may default to using 'ascii' encoding.  Since the documentation is in
UTF-8, and contains characters beyond 'ascii', this will result in
failures when using these versions of Python.

Fix this by being explicit about the encoding to be used when reading
these files.

Signed-off-by: David Brown <david.brown@linaro.org>
Change-Id: I54c69334c6bf377f1135cec04f4e0ea96a8e9a5b