Docs refer to OpenVPN files which have moved in newer versions

According to https://github.com/OpenVPN/openvpn/blob/master/distro/systemd/README.systemd, as of OpenVPN v2.4, most/all configuration files were moved from /etc/openvpn/ to /etc/openvpn/server/.

A number of the pihole guide pages still refer to the old path:

docs/docs/guides/vpn/dual-VPN.md

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           You should have an existing OpenVPN server configured and running. We are going to use the original configuration file located at `/etc/openvpn/server.conf`.  
        
           First, copy the file: 
        
           ```bash 
        
           sudo cp /etc/openvpn/server.conf /etc/openvpn/server2.conf

docs/docs/guides/vpn/dynDNS.md

Line 4 in 4a6bb6c

vim /etc/openvpn/client-common.txt

docs/docs/guides/vpn/setup-openvpn-server.md

Line 17 in 4a6bb6c

vim /etc/openvpn/server.conf

docs/docs/guides/vpn/only-dns-via-vpn.md

Line 5 in a7d259a

    
           Edit your `/etc/openvpn/server.conf` and remove (comment out) the following line:

docs/docs/guides/vpn/dual-operation.md

Line 12 in 95956c3

Edit your `/etc/openvpn/server.conf`:

... and more (listing them is tedious when it's easy to fix).

Caused me a bit of confusion when I first ran through the guide - spent about an hour scratching my head as to why my settings weren't applying or things were missing until I realised everything was under the server folder.

I'll raise a PR to fix this issue.

	You should have an existing OpenVPN server configured and running. We are going to use the original configuration file located at `/etc/openvpn/server.conf`.

	First, copy the file:

	```bash
	sudo cp /etc/openvpn/server.conf /etc/openvpn/server2.conf