guide: revert the anchor "fix" -- it wasn't broken, now verified
Forgejo's markdown renderer auto-prefixes *all* fragment links with
user-content-, not just heading ids -- confirmed by checking the actual
rendered href this time instead of assuming from the heading id alone.
The plain #chat-auf-dem-eigenen-gerät link was already correct; manually
adding the prefix double-stacked it into
#user-content-user-content-chat-auf-dem-eigenen-gerät.
Turn the guide's rough outline into an actual step-by-step walkthrough
Nothing removed -- the numbered outline (plaza -> account -> lldap ->
password -> universal login -> chat -> agent) becomes a proper
"Loslegen" section with one heading per step, absorbing the old "Erste
Schritte" (lldap login) and folding the chat login details into steps
6-7. "Chat beitreten" survives as "Chat auf dem eigenen Gerät", now
specifically the native-app path referenced from step 6, since the web
chat is already covered there. Everything else (Fediverse, Öffentliche
Lobby, Vikunja, Kalender, Website, Intern) is untouched.
