Add whitespace task and fix trailing whitespace
While running the docs monthly tasks here: gitlab-org/technical-writing#712 (closed)
The whitespace task picked up some whitespace issues: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3309811829
There is no task that can automatically update these for you, so adding this here too. Similar to https://gitlab.com/gitlab-org/security/charts/gitlab/-/tree/master/doc/.markdownlint