docs: rewrite README with structured overview and quick start guide

Replaces the minimal project description with a comprehensive README
including a component overview table, quick start instructions, common
Ansible operations, and links to detailed documentation. Aligns with
Red Panda Approval™ standards.

ansible/roles/incus_storage_bucket/defaults/main.yml

@@ -0,0 +1,17 @@
+---
+# Default variables for incus_storage_bucket role
+
+# Incus configuration (should match terraform.tfvars)
+storage_pool: default
+project_name: agathos
+bucket_role: admin
+
+# Service-specific variables (must be provided)
+# bucket_name: casdoor
+# service_name: casdoor
+
+# Path to S3 credentials vault file (separate from main vault)
+s3_credentials_file: "{{ playbook_dir }}/inventory/group_vars/all/s3_vault.yml"
+
+# Task selection
+task_action: create  # create, regenerate, or remove