docs: add usage in Coder (#5975)

* docs: add difference between Coder

Add a short block explaining the difference between code-server and
Coder.

* docs: add new doc coder.md under Install

This adds a new doc explaining how to install code-server in a Coder
workspace using Terraform.

docs/coder.md

@@ -0,0 +1,37 @@
+# Coder
+
+To install and run code-server in a Coder workspace, we suggest using the `install.sh`
+script in your template like so:
+
+```terraform
+resource "coder_agent" "dev" {
+  arch           = "amd64"
+  os             = "linux"
+  startup_script = <<EOF
+    #!/bin/sh
+    set -x
+    # install and start code-server
+    curl -fsSL https://code-server.dev/install.sh | sh -s -- --version 4.8.3
+    code-server --auth none --port 13337 &
+    EOF
+}
+
+resource "coder_app" "code-server" {
+  agent_id     = coder_agent.dev.id
+  slug         = "code-server"
+  display_name = "code-server"
+  url          = "http://localhost:13337/"
+  icon         = "/icon/code.svg"
+  subdomain    = false
+  share        = "owner"
+
+  healthcheck {
+    url       = "http://localhost:13337/healthz"
+    interval  = 3
+    threshold = 10
+  }
+}
+```
+
+If you run into issues, ask for help on the `coder/coder` [Discussions
+here](https://github.com/coder/coder/discussions).