This is a pre-release commit for the Yoga release following our docs [1]
* No API additions during this cycle (no need to update
doc/source/contributor/webapi-version-history.rst)
* Add prelude releasenote
* Fix highlight in one releasenote
[1] https://docs.openstack.org/ironic/latest/contributor/releasing.html
Change-Id: If26019c5abc0643051f4b11186d7eb819f9c407b