feat: add branding — favicon, logo header, and apple-touch-icon

- Copy favicon.ico, favicon-32x32.png, apple-touch-icon.png, logo-128.png to assets/branding/
- Add favicon links to _includes/head_custom.html
- Add centered logo to index.md landing page header

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: emmanuelknafo

_includes/head_custom.html

@@ -1,3 +1,6 @@
+<link rel="icon" type="image/x-icon" href="{{ site.baseurl }}/assets/branding/favicon.ico">
+<link rel="icon" type="image/png" sizes="32x32" href="{{ site.baseurl }}/assets/branding/favicon-32x32.png">
+<link rel="apple-touch-icon" sizes="180x180" href="{{ site.baseurl }}/assets/branding/apple-touch-icon.png">
 <script type="module">
   import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.esm.min.mjs';
   mermaid.initialize({ startOnLoad: false });

assets/branding/apple-touch-icon.png

@@ -5,6 +5,10 @@ nav_order: 0
 permalink: /
 ---
 
+<p align="center">
+  <img src="assets/branding/logo-128.png" alt="Agentic Accelerator Framework" width="100">
+</p>
+
 # FinOps Cost Governance Workshop
 
 Welcome to the **FinOps Cost Governance Workshop** — a hands-on, progressive workshop that teaches you how to scan Azure infrastructure for cost governance violations using four open-source tools: PSRule, Checkov, Cloud Custodian, and Infracost.