Deprecated: Move reference architectures components to a new location

With the consolidation of the HA and scaling docs #215161 (closed), there's no need to keep the components in the high_availability dir. They should be moved under administration/.

Currently:

high_availability
├── README.md
├── alpha_database.md
├── consul.md
├── database.md
├── gitaly.md
├── gitlab.md
├── img
│   ├── fully-distributed.png
│   ├── geo-ha-diagram.png
│   ├── horizontal.png
│   ├── hybrid.png
│   └── pg_ha_architecture.png
├── load_balancer.md
├── monitoring_node.md
├── nfs.md
├── nfs_host_client_setup.md
├── pgbouncer.md
├── redis.md
├── redis_source.md
└── sidekiq.md

Migrate

• consul.md !31728 (closed)
• load_balancer.md
• sidekiq.md (already have an epic to update the docs)
• gitlab.md
• monitoring_node.md
• gitaly.md (already have an epic to update the docs)
• nfs.md
  • nfs_host_client_setup.md
• redis.md (already have an epic to update the docs)
• redis_source.md
• database.md (already have an epic to update the docs)
  • pgbouncer.md

1. Copy old doc to new location:

   mkdir -pv doc/administration/consul
   cp doc/administration/high_availability/consul.md doc/administration/consul/index.md

2. Change the content of the old doc to redirect to the new one https://docs.gitlab.com/ee/development/documentation/#changing-document-location.
3. Fix all links.
4. Commit.
5. Change the navigation item in the docs site.
6. Repeat for all items.

Some of them will need to be renamed, for example gitlab.md should be gitlab_rails.md to be more clear what this is about. We might also need to merge the two NFS docs we have.