ci: doc: Publish pull request docs to builds.zephyrproject.io

This commit updates the CI documentation build workflow to upload the
HTML pull request documentation builds to the S3 builds.zephyrproject.io
bucket so that they are directly accessible from the web.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
(cherry picked from commit 1dd92ec8)