doc: Use Gitaly documentation as source of truth

As part of https://gitlab.com/gitlab-org/gitaly/-/issues/6388, we've
identified several places where Gitaly's architecture is documented.

Customers and customer support are more likely to read about Gitaly
through the official documentation rather than this README. There's no
point in maintaining the same information in both places.

Remove the architecture diagrams and list of Gitaly clients from the
README, and add a link to the documentation instead.

README.md

@@ -51,67 +51,8 @@ GitLab end up in Gitaly.
 
 ## Design
 
-High-level architecture overview:
-
-```mermaid
-graph LR
-
-  subgraph "Gitaly Service"
-  Gitaly == git ==> Filesystem
-  end
-
-  subgraph "Clients"
-    Rails[gitlab-rails] --> Gitaly
-    Workhorse --> Gitaly
-    Shell[gitlab-shell] -- command-line\nclient --> Gitaly
-    Gitaly -. Authorization .-> Rails
-  end
-```
-
-In [High Availability](#high-availability) mode, the current implementation looks like this (some details omitted):
-
-```mermaid
-graph LR
-
-  subgraph "Gitaly Nodes"
-  Gitaly == git ==> Filesystem
-  end
-
-  subgraph "Praefects"
-    LB[typical setup uses a loadbalancer] --> P1
-    LB --> P2
-    P1[Praefect 1]
-    P2[Praefect N]
-    P1 --> PG[(PostgreSQL)]
-    P2 --> PG
-  end
-
-  subgraph "Clients"
-    Rails[gitlab-rails]
-    Workhorse
-    Shell[gitlab-shell]
-  end
-
-Clients --> Praefects --> Gitaly
-```
-
-### Gitaly clients
-
-As of Q4 2018, the following GitLab components act as Gitaly clients:
-
-- [`gitlab`](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/gitaly_client.rb):
-  the main GitLab Rails application.
-- [`gitlab-shell`](https://gitlab.com/gitlab-org/gitlab-shell/tree/main):
-  for `git clone`, `git push` etc. via SSH.
-- [`gitlab-workhorse`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/workhorse/internal/gitaly/gitaly.go):
-  for `git clone` via HTTPS and for slow requests that serve raw Git data.
-- [`gitaly-ssh`](https://gitlab.com/gitlab-org/gitaly/tree/master/cmd/gitaly-ssh):
-  for internal Git data transfers between Gitaly servers.
-
-The clients written in Go (`gitlab-shell`, `gitlab-workhorse`, `gitaly-ssh`)
-use library code from the
-[`gitlab.com/gitlab-org/gitaly/client`](https://gitlab.com/gitlab-org/gitaly/tree/master/client)
-package.
+The [Gitaly and Gitaly cluster](https://docs.gitlab.com/ee/administration/gitaly/) documentation details the architecture
+and design of Gitaly, including a list of known Gitaly consumers.
 
 ## High Availability