Start using tags to mark stable docs releases

As a POC, I created a few tags and updated the docs-lint image to use the docs.latest.stable tag. This way, the docs-lint image will always use the latest stable version of the docs project.

In testing, this caused a few pipelines to spawn when the tags were created, see https://gitlab.com/gitlab-org/gitlab-docs/pipelines/107165002 for an example. As a result, I added a tag exception so that image:builder-onbuild and image:nginx-onbuild don't run on tag creation, and I also forced compile_dev to run on tag creation, so that we can run the full suite of tests on the tag, to make sure it's a good commit to use for the tag.