Convert legacy documentation to Markdown

## Goal

Rewrite the collected material in `legacy/` as VuePress Markdown pages in `docs/`. The result must read like one handbook written by one author, not like a pile of copied articles.

Target readers are administrators who run a DWH at a hospital. They know Linux basics, they do not know AKTIN internals.

## Rules for every page

Follow what the finished pages in `docs/install/` and `docs/update/` already do:

- Language: German, formal address (Sie). Technical terms stay English.
- One H1 per file, it is the page title. Steps are numbered H2 headings (`## 1. Vorbereitung`).
- Images go to `docs//images/` and are referenced relatively: ``. Give files a descriptive name (`requests3.png`, not `Screenshot from 2026-01-21.png`). Crop screenshots to the relevant area.
- Commands in fenced code blocks with a language tag (```` ```bash ````). No `sudo`, the docs assume a root shell (see `docs/install/debian-install.md`).
- Version numbers via `{{ $theme.versions.dwh }}`, `{{ $theme.versions.ubuntu }}`, `{{ $theme.versions.codename }}`. Values come from `.env`. Never hardcode.
- Links used on several pages and links to external resources are defined once in `docs/.vuepress/_refs.md` and used as `[Text][ref-name]`. Check that the target exists. `[i2b2-operation]` currently points to a page that does not exist.
- Use containers for hints: `::: tip`, `::: warning`, `::: danger`, `::: info`. Use `::: tabs` when Debian and Docker differ.
- Every new page must be added to the sidebar in `docs/.vuepress/config.js`, otherwise it is not reachable.
- Delete the legacy source from `legacy/` in the same pull request once the page is converted.
- Do not just translate the old text. Check every command and screenshot against the current DWH version and remove anything that is obsolete.