arm-bsp/documentation: corstone1000: Install Sphinx theme as recommended

Read the Docs recommends installing the Sphinx theme by listing
it as an enabled extensions prior to setting it as the active theme.

This commit adds it to the enabled extensions list as it was already
set as the active theme.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-bsp/documentation/corstone1000/conf.py

@@ -32,6 +32,7 @@ author = 'Arm Limited'
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
+    'sphinx_rtd_theme',
 ]
 
 # Add any paths that contain templates here, relative to this directory.