Misc doc fixes (#2562)

* Link to stable and development docs in the README

* Add Tailscale SSH and autogroup:nonroot to features page

* Use @ when referencing users in policy

* Remove unmaintained headscale-webui

The project seems to be unmaintained (last commit: 2023-05-08) and it
only supports Headscale 0.22 or earlier.

* Use full image URL in container docs

This makes it easy to switch the container runtime from docker <->
podman.

* Remove version from docker-compose.yml example

This is now deprecated and yields a warning.

docs/about/features.md

@@ -22,10 +22,12 @@ provides on overview of Headscale's feature and compatibility with the Tailscale
 - [x] Embedded [DERP server](https://tailscale.com/kb/1232/derp-servers)
 - [x] Access control lists ([GitHub label "policy"](https://github.com/juanfont/headscale/labels/policy%20%F0%9F%93%9D))
     - [x] ACL management via API
-    - [x] Some [Autogroups](https://tailscale.com/kb/1396/targets#autogroups), currently: `autogroup:internet`
+    - [x] Some [Autogroups](https://tailscale.com/kb/1396/targets#autogroups), currently: `autogroup:internet`,
+      `autogroup:nonroot`
     - [x] [Auto approvers](https://tailscale.com/kb/1337/acl-syntax#auto-approvers) for [subnet
       routers](../ref/routes.md#automatically-approve-routes-of-a-subnet-router) and [exit
       nodes](../ref/routes.md#automatically-approve-an-exit-node-with-auto-approvers)
+    - [x] [Tailscale SSH](https://tailscale.com/kb/1193/tailscale-ssh)
 * [ ] Node registration using Single-Sign-On (OpenID Connect) ([GitHub label "OIDC"](https://github.com/juanfont/headscale/labels/OIDC))
     - [x] Basic registration
     - [x] Update user profile from identity provider