The .deb install instructions in the README pointed at a URL that
doesn't exist — Gitea exposes the Debian registry as an apt repo, not
as plain file downloads. Switched the README to the apt-repo flow
(add a sources.list line, then apt install).
Also fixed build-deb.sh: Gitea's Debian package endpoint returns
HTTP 405 for token-bearer auth; it requires HTTP basic auth (user +
token-as-password) and the literal /upload suffix on the URL.
Package built and pushed to the registry — apt install works now.
Lay out the three install paths (Windows installer, .deb package, manual
docker compose) with concrete numbered steps and a 'pick your method'
table at the top so users don't have to read past their own platform.
Add a using-it walkthrough, a scan-mode explanation, and a short
troubleshooting section.
