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/raspberrypi/meta/main.yml

@@ -0,0 +1,10 @@
+---
+argument_specs:
+  main:
+    short_description: Adjust some Raspberry Pi specific requisites
+    options:
+      state:
+        default: present
+        description:
+          - Indicates whether the k3s prerequisites for Raspberry Pi should be
+          - set up (possible values are `present` and `absent`)