[DOC] Add configuration template for Vikunja

Vikunja can be reverse proxied by BunkerWeb like this:

```yml
version: '3'

services:
db:
image: mariadb:10
networks:
- vikunja_network
command: --character-set-server=utf8mb4 --collation-server=utf8mb4_unicode_ci
environment:
MYSQL_ROOT_PASSWORD: supersecret
MYSQL_USER: vikunja
MYSQL_PASSWORD: secret
MYSQL_DATABASE: vikunja
volumes:
- ${CONFIG_FOLDER}/vikunja/mysql:/var/lib/mysql
restart: unless-stopped
api:
image: vikunja/api
networks:
- vikunja_network
- services
environment:
VIKUNJA_DATABASE_HOST: db
VIKUNJA_DATABASE_PASSWORD: secret
VIKUNJA_DATABASE_TYPE: mysql
VIKUNJA_DATABASE_USER: vikunja
VIKUNJA_DATABASE_DATABASE: vikunja
VIKUNJA_SERVICE_JWTSECRET: supersecret
VIKUNJA_SERVICE_FRONTENDURL: https://vikunja.youtdomain.com/
volumes:
- ${CONFIG_FOLDER}/vikunja/api:/app/vikunja/files
- ${CONFIG_FOLDER}/vikunja/config:/.config
depends_on:
- db
restart: unless-stopped
labels:
- "bunkerweb.SERVER_NAME=vikunja.youtdomain.com"
- "bunkerweb.ALLOWED_METHODS=GET|POST|HEAD|DELETE|PUT"
- "bunkerweb.REVERSE_PROXY_URL=/"
- "bunkerweb.REVERSE_PROXY_HOST=http://vikunja-frontend-1:80"
- "bunkerweb.REVERSE_PROXY_URL_1=~* ^/(api|dav|[.]well[-]known)/"
- "bunkerweb.REVERSE_PROXY_HOST_1=http://vikunja-api-1:3456"
- "bunkerweb.MAX_CLIENT_SIZE_1=20M"
- "bunkerweb.REVERSE_PROXY_WS=yes"
- "bunkerweb.REVERSE_PROXY_WS_1=yes"

frontend:
image: vikunja/frontend
restart: unless-stopped
networks:
- vikunja_network
- services

networks:
vikunja_network:
name: vikunja
ipam:
driver: default
config:
- subnet: 172.22.0.0/24
services:
name: services
external: true
```

No file or test is named in the issue. Start by locating the repository's existing configuration-template conventions and compare this Docker Compose example with them. Done means a Vikunja template is added in the established location and matches the supplied reverse-proxy settings.

Written by the indexing model from the issue text.