Variable documentation (#606)

* docs: first modules' variable docs table

* docs: variables for k3s_server_post

* docs: lxc and prereq vars in README

* style: lint errors

* docs: argument_specs for proxmox_lxc

* docs: last variables found added to the README

roles/proxmox_lxc/meta/main.yml

@@ -0,0 +1,9 @@
+---
+argument_specs:
+  main:
+    short_description: Proxmox LXC settings
+    options:
+      proxmox_lxc_ct_ids:
+        description: Proxmox container ID list
+        type: list
+        required: true