docs: simplify compose example (#10407)

# Which Problems Are Solved

Using the compose configuration with for existing installation doesn't
work.
It is not appreciated in the whole community, that we use Traefik for
the deployment example.

# How the Problems Are Solved

- The config we used before v4 is reused and extended in a compatible
way.
- Traefik is removed, Zitadel is accessible again at localhost:8080 and
the login at localhost:3000. Deploying with Traefik is described already
here http://localhost:3000/docs/self-hosting/manage/reverseproxy/traefik
- A page is added that describes how to create a login client and switch
from login v1 to v2. The page is linked in a hint in the compose
example.

# Additional Changes

- The config also sets up a machine user and documents it inline. This
allows us to avoid using the cumbersome separate docker-compose-sa.yaml

# Additional Context

- Closes #10379 
- Internal discussion
https://zitadel.slack.com/archives/C08TL9AURL7/p1754471184222879
- Discord about upgrading to v2 login:
-
https://discord.com/channels/927474939156643850/927866013545025566/threads/1401950163940933804
-
https://discord.com/channels/927474939156643850/927866013545025566/1401312126030708756
- Discord about not found errors:
-
https://discord.com/channels/927474939156643850/927866013545025566/threads/1401173877941473291
-
https://discord.com/channels/927474939156643850/927866013545025566/1401045717849604227
-
https://discord.com/channels/927474939156643850/927866013545025566/1401173877941473291
-
https://discord.com/channels/927474939156643850/927866013545025566/1401301168998584361

docs/sidebars.js

@@ -1094,6 +1094,7 @@ module.exports = {
       items: [
         "self-hosting/manage/production",
         "self-hosting/manage/productionchecklist",
+        "self-hosting/manage/login-client",
         "self-hosting/manage/configure/configure",
         {
           type: "category",