Update the branch pattern used for docs related Pipelines

In few of our projects, we're using a special branch naming for docs related changes: https://docs.gitlab.com/ce/development/documentation/#branch-naming. This allows us to start only tests related to docs, and not execute any other CI/CD Jobs, when only docs are updated. It makes the testing way faster and saves our resources.

At this moment the expected pattern of the branch is docs-..., ...-docs or docs/.... As it was discussed in gitlab-runner!1113 (merged) it would be nice to extend this an allow also docs_... and ..._docs since using underscores instead of dashes is also a popular pattern.

If we decide to do this, we should update this in all projects where the naming and is supported in the CI/CD configuration:

• https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/.gitlab-ci.yml
• https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/.gitlab-ci.yml