[docs] Retire openstack/docs-specs

Hey, I'm proposing retiring docs-specs [1][2]. This hasn't been used in some time since most of our documentation now lives in individual project repositories and any work that would span these multiple projects would likely exist as a community goal, documented in those relevant repositories. I do not intend for the existing content to be removed - it simply won't be possible to add new specs. Let me know if anyone has a reason not to do this. If not, we'll get a move on with this next week. Stephen [1] https://opendev.org/openstack/docs-specs [2] https://review.opendev.org/#/c/668853/