Update Hugo build test jobs to use the same branch (version) as their product

The hugo_build CI jobs in our upstream content projects check that Hugo will still build when docs are updated. They check for issues like malformed shortcodes or invalid file names.

Example job: https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/ci/docs.gitlab-ci.yml?ref_type=heads#L116

These tests clone the main branch of the Docs website project, but if certain dependencies for running builds change (like in the addition of gum in !598 (merged)), that main Docs branch might not work for older versions of gitlab/runner/etc any longer; the environment and the code may no longer be compatible.

We need to have the Docs site checkout match the version of the product that's cloning it.

Note that the Docs site does not have patch versions as branches, so we need to clone the Docs site at the applicable xx.yy version.

Operator versions don't map to GitLab versions very easily. The Docs site always builds Operator docs from its default branch rather than stable release branches. Might make sense to just omit that one.

Edited by Evan Read