Rewrite ACL docs as policy

- Rename from acl.md to policy.md and setup redirect links
- Mention both ACLs and Grants
- Remove most old ACL docs and replace with links to Tailscale docs
- Add "Getting started" section
- Add section about notable differences

docs/ref/registration.md

@@ -61,8 +61,8 @@ headscale users create <USER>
 === "Tagged devices"
 
     Your Headscale user needs to be authorized to register tagged devices. This authorization is specified in the
-    [`tagOwners`](https://tailscale.com/docs/reference/syntax/policy-file#tag-owners) section of the [ACL](acls.md). A
-    simple example looks like this:
+    [`tagOwners`](https://tailscale.com/docs/reference/syntax/policy-file#tag-owners) section of the
+    [policy](policy.md). A simple example looks like this:
 
     ```json title="The user alice can register nodes tagged with tag:server"
     {