[DOC] High availablity via keepalived

### Description

I was able to get a proper HA setup without any additional loadbalancer in front of a Bunkerweb instance.
To do so I installed keepalived on my two Ubuntu VMs with following config:

##VM1
```json
! Configuration File for keepalived

global_defs {

vrrp_script chk_docker {
script "/etc/keepalived/docker.sh bunkerweb" # verify the pid existance
interval 2 # check every 2 seconds
weight 2 # add 2 points of prio if OK
}

vrrp_instance VI_1 {
state MASTER
interface eth0
virtual_router_id 102
priority 111
advert_int 1
authentication {
auth_type PASS
auth_pass 4Ludwh3w
}
virtual_ipaddress {
10.0.20.19/24
}
track_script {
chk_docker
}
}
```

On VM2 I set `priority 110` so it stays as a backup.

The docker.sh script would be:
```bash
#!/bin/bash
if [ $(docker inspect --format="{{.State.Restarting}}" $1) = "false" ] && [ $(docker inspect --format="{{.State.Running}}" $1 2> /dev/null) = "true" ]; then
exit 0;
else
exit 1;
fi
```

The only thing that's currently missing is to have kind of a check as soon as Bunkerweb is fully available meaning it can handle traffic.
Running/healthy Bunkerweb is pulling the current config from the Bunkerweb scheduler before it starts handling traffic.

### Proposed solution (optional)

See abovr

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

No repository file or test is named. Start by reviewing the keepalived configuration and docker.sh example, then trace when Bunkerweb finishes pulling configuration from the scheduler and can handle traffic. Done means the HA check distinguishes a running container from a traffic-ready Bunkerweb instance and the setup is documented.

Written by the indexing model from the issue text.