Docs: Clarify cluster management methods in the docs

Follow-up from the "Kubernetes cluster documentation" meeting on 2021-10-19. Agenda.

Move deprecated/to-deprecate features into a separate nav section => &6499 (comment 710517787) => gitlab-docs!2211 (merged)
Put the Agent under "Management" => slightly different proposal here => &6499 (comment 710517787) => gitlab-docs!2211 (merged)
Phrase the certificate-based features to "still supported, but may be deprecated in a future version of GitLab" => I'll use the same language used here: gitlab-com/www-gitlab-com!92391 (closed) => Done by !72896 (merged)
Add a warning to all "pre-deprecated" features => #343794 (closed)

UPDATE: see gitlab-com/www-gitlab-com!92391 (closed)

Edited Nov 08, 2021 by Marcia Ramos