Removes and redirects cells design documents

What does this MR do?

❗ Adds cells design documents and notes partitioning (gitlab-com/content-sites/handbook!7085 - merged) is a MR dependency for this MR ❗ We should only merge this once the dependency has been deployed!

We are currently migrating the Architecture design documents from the gitlab project to the public handbook.

gitlab-com/content-sites/handbook!7085 (merged) adds the cells design documents to the handbook (plus a hidden design doc for notes table partitioning) and missing redirects for backup_and_restore and secret_manager. See table on the migration issue). Now this MR removes the original documents from the gitlab repository and instead adds redirects to the handbook. It also adds a redirect to the main "old" blueprints page. This MR wraps up the design documents migration.

Excerpt from design docs table. These documents are in this MR.

Design document folder	Creation Date	Migrated?	Redirect?
cells	2022-09-07	❗ Open MRs !148010 !146768 (merged) !156678 (merged) !151365 👷 Separate MR for cells migration
notes_table_partitioning	2023-12-22	👷 Wasn't listed on the blueprints page because it wasn't in a separate folder.
backup_and_restore		gitlab-com/content-sites/handbook!7085 (merged)
secret_manager			Redirect for index.md was missing

Related issues

Move Architecture Blueprints to Public Handbook (gitlab-com/content-sites/handbook#279 - closed)

Author's checklist

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