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

mkdocs.yml

@@ -77,7 +77,8 @@ plugins:
   - social: {}
   - redirects:
       redirect_maps:
-        acls.md: ref/acls.md
+        acls.md: ref/policy.md
+        ref/acls.md: ref/policy.md
         android-client.md: usage/connect/android.md
         apple-client.md: usage/connect/apple.md
         dns-records.md: ref/dns.md
@@ -184,7 +185,7 @@ nav:
       - OpenID Connect: ref/oidc.md
       - Routes: ref/routes.md
       - TLS: ref/tls.md
-      - ACLs: ref/acls.md
+      - Policy: ref/policy.md
       - DNS: ref/dns.md
       - DERP: ref/derp.md
       - API: ref/api.md