Add global operator config options docs

What does this MR do?

Adds docs for Allow to to configure all global config.toml se... (gitlab-org/gl-openshift/gitlab-runner-operator!175 - merged) • Georgi N. Georgiev | GitLab • 17.9

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Related to Allow configuring all global config.toml options (gitlab-org/gl-openshift/gitlab-runner-operator#234 - closed) • Georgi N. Georgiev | GitLab • 17.9