generate yang docs (with improved css)

[hellt]

the oc-yang plugin can generate nice html docs for yang models as shown here
http://ops.openconfig.net/branches/models/telemetry-version/docs/openconfig-acl.html

we can improve the css styling and make it a beautifully looking add-on to this yang browser

https://github.com/openconfig/oc-pyang

pyang --plugindir ${YANG_PLUGINS} -f docs --doc-format html [PATH_TO_YANG_FILES]

https://github.com/openconfig/oc-pyang/blob/master/openconfig_pyang/plugins/yangdoc.py

/cc @siva19susi