Remove instrumentation page from left nav
I traced this failure https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines/374627432/failures back to gitlab!70533 (merged) where the page was removed. I'm unsure why the pipelines for that MR were green; shouldn't they have flagged the link in the left nav? (@axil, @marcel.amirault)
Either way, let's fix the nav.