Fix instructions for merging release MR

As discussed in #1274 (comment 1111663096), the steps for deployment were the wrong way round. This MR updates the steps so that the MR is merged before the docker image is built. It also includes a step to manually kick off the deployment pipeline so you don't have to wait for the next scheduled one.

This also aligns with the instructions in the docs: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/releases.md#merge-merge-requests-and-run-docker-image-builds