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/prometheus/alertmanager_deploy.yml

@@ -0,0 +1,48 @@
+---
+- name: Deploy Prometheus Alertmanager with Pushover
+  hosts: ubuntu
+  become: true
+  tasks:
+  - name: Check if host has alertmanager service
+    ansible.builtin.set_fact:
+      has_alertmanager_service: "{{'alertmanager' in services}}"
+
+  - name: Skip hosts without alertmanager service
+    ansible.builtin.meta: end_host
+    when: not has_alertmanager_service
+
+  - name: Install Alertmanager
+    ansible.builtin.apt:
+      name: prometheus-alertmanager
+      state: present
+      update_cache: true
+
+  - name: Create alertmanager config directory
+    ansible.builtin.file:
+      path: /etc/alertmanager
+      state: directory
+      owner: prometheus
+      group: prometheus
+      mode: '750'
+
+  - name: Template alertmanager configuration
+    ansible.builtin.template:
+      src: prometheus/alertmanager.yml
+      dest: /etc/alertmanager/alertmanager.yml
+      owner: prometheus
+      group: prometheus
+      mode: '550'
+    notify: restart alertmanager
+
+  - name: Start and enable Alertmanager service
+    ansible.builtin.systemd:
+      name: prometheus-alertmanager
+      state: started
+      enabled: true
+      daemon_reload: true
+
+  handlers:
+  - name: restart alertmanager
+    ansible.builtin.systemd:
+      name: prometheus-alertmanager
+      state: restarted