Set up VuePress config

## Goal

Provide the site skeleton so that content authors (#3) only need to write Markdown. This issue covers everything in `docs/.vuepress/` and the repository `README.md`.

## Current state

The skeleton works. `npm run docs:dev` serves the site with sidebar, search, footer and version variables. What is still missing is listed under Open items.

Files involved:

- `docs/.vuepress/config.js`: site metadata, sidebar, search plugin, global link references, version variables read from `.env`
- `docs/.vuepress/_refs.md`: link references available in every page
- `docs/.vuepress/client.js` and `docs/.vuepress/layouts/Layout.vue`: extends the default theme layout and adds the footer
- `docs/.vuepress/public/images/`: logo and favicon, served as `/images/logo.png` and `/images/signet.png`
- `.env`: `DWH_VERSION`, `API_VERSION`, `UBUNTU_VERSION`, `UBUNTU_CODENAME`

Constraint: stay on `@vuepress/theme-default`. Do not add the Hope theme, it pulls in far more than we need.

- [x] Site metadata (title, description, `lang: de-DE`) in `config.js`
- [x] Sidebar with the four sections
- [x] Logo and favicon files in `public/images/`
- [x] Site title shows the DWH version from `.env`
- [x] Version variables usable in Markdown as `{{ $theme.versions.dwh }}`
- [x] Global search bar (`@vuepress/plugin-search`)
- [x] Global link references from `_refs.md` injected into every page
- [x] Footer with copyright and CC BY 4.0 notice (`Layout.vue`)
- [ ] Footer should always sit at the bottom of the viewport, also on short pages. It is currently rendered through the `page-bottom` slot and moves up when the content is short. Reference for the look: https://theme-hope.vuejs.press/demo/ (look only, do not install).
- [ ] The favicon `` in `config.js` `head` points to `.vuepress/public/images/signet.png`. Files in `public/` are served from the site root, so the href must be `/images/signet.png`. Verify in the browser tab.
- [ ] Decide on `base` in `config.js`. GitHub Pages serves project sites under `https://aktin.github.io/aktin-github-handbook/`, which needs `base: '/aktin-github-handbook/'`. With a custom domain `base` stays `/`. Coordinate with #5.
- [ ] PDF export of a page (nice to have). Check first whether the browser print view of the default theme is good enough. Only look for a plugin if it is not.
- [ ] Extend `README.md`: what the repository is, how to run it locally, how to add a page, how deployment works. Keep it short and link to #1 for details.