Move sub-blueprints to cells-*

What does this MR do?

Briefly describe Move sub-blueprints to cells-*

Contents not updated, will be done in follow-up MR

Done using the following script:

script

require 'fileutils'

def copy(old, new)
  FileUtils.cp(old, new)
end

def create_redirect(old, new)
  system %Q{bundle exec rake "gitlab:docs:redirect[#{old}, #{new}]"}
end

DIR = File.expand_path('doc/architecture/blueprints/cells', __dir__)

Dir[File.join(DIR, 'pods-*.md')].each do |old_path|
  new_path = File.join(DIR, File.basename(old_path).sub('pods-', 'cells-'))

  copy(old_path, new_path)
  create_redirect(old_path, new_path)
end

=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-admin-area.md to cells-feature-admin-area.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-agent-for-kubernetes.md to cells-feature-agent-for-kubernetes.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-backups.md to cells-feature-backups.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-ci-runners.md to cells-feature-ci-runners.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-container-registry.md to cells-feature-container-registry.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-contributions-forks.md to cells-feature-contributions-forks.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-dashboard.md to cells-feature-dashboard.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-data-migration.md to cells-feature-data-migration.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-database-sequences.md to cells-feature-database-sequences.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-git-access.md to cells-feature-git-access.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-gitlab-pages.md to cells-feature-gitlab-pages.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-global-search.md to cells-feature-global-search.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-graphql.md to cells-feature-graphql.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-organizations.md to cells-feature-organizations.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-personal-namespaces.md to cells-feature-personal-namespaces.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-router-endpoints-classification.md to cells-feature-router-endpoints-classification.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-schema-changes.md to cells-feature-schema-changes.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-secrets.md to cells-feature-secrets.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-snippets.md to cells-feature-snippets.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-template.md to cells-feature-template.md
=> Creating new redirect from /Users/tkuah/code/gdk-ee/gitlab/doc/architecture/blueprints/cells/pods-feature-uploads.md to cells-feature-uploads.md

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