docs: add wireguard docs

Signed-off-by: Leonidas Spyropoulos <artafinde@archlinux.org>

docs/wireguard.md

+# WireGuard
+
+Many of our servers communicate through wireguard VPN with each others. If you need to collect logs with `loki` and metrics with `prometheus` for dashboards you need to have a wiregauard IP.
+
+## Setting up
+1. For a new server add a new unused wireguard IP and set the following in `host_vars/<fqdn>/misc`
+    ```
+    wireguard_address: <wg-ip>
+    wireguard_public_key: <wg-pubkey>
+    ```
+
+1. Save the private key in a encypted vault in  `host_vars/<fqdn>/vault_wireguard.yml`
+
+    Tips: 
+    - Pick next available IP for Wireguard from `grep -r wireguard_address host_vars/ | cut -f3 -d: | sort -h`
+
+    - Wireguard key generation docs: https://www.wireguard.com/quickstart/#key-generation 
+1. Execute `wireguard` and `prometheus` roles on `monitoring.archlinux.org.yml` playbook to get data from the server