docs: use Spec Kit branding on documentation site (#4264)

Use the README logo for the DocFX navbar, favicon, and landing hero, and add Upgrade to the balanced Explore the docs grid.

Assisted-by: GitHub Copilot (model: GPT-5.6 Sol, autonomous)

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

Copilot-Session: 78ca683f-2995-44eb-a2fa-f7600c18bbd9

Author: mnriem

docs/docfx.json

@@ -64,6 +64,8 @@
     "globalMetadata": {
       "_appTitle": "Spec Kit Documentation",
       "_appName": "Spec Kit",
+      "_appLogoPath": "images/spec-kit-logo.webp",
+      "_appFaviconPath": "images/spec-kit-logo.webp",
       "_appFooter": "Spec Kit - A specification-driven development toolkit",
       "_enableSearch": true,
       "_disableContribution": false,

docs/images/spec-kit-logo.webp

@@ -1,5 +1,7 @@
 <div class="landing-hero">
 
+<img class="landing-hero-logo" src="images/spec-kit-logo.webp" alt="" width="120" height="120">
+
 # GitHub Spec Kit
 
 **Spec-Driven Development or your own process — step by step or as an automated workflow.**
@@ -128,6 +130,10 @@ Community extensions like CI Guard and Architecture Guard add compliance gates a
     <strong>Existing Projects</strong>
     <span>Adopt Spec Kit safely in an established codebase</span>
   </a>
+  <a href="upgrade.md" class="nav-card">
+    <strong>Upgrade</strong>
+    <span>Keep an existing Spec Kit project current across releases</span>
+  </a>
   <a href="reference/overview.md" class="nav-card">
     <strong>Reference</strong>
     <span>Core commands, integrations, extensions, presets, and workflows</span>

docs/index.md

@@ -25,6 +25,13 @@
   --gh-coral-subtle: #2d0f0d;
 }
 
+/* Keep the raster Spec Kit logo aligned with DocFX's default header dimensions. */
+.navbar-brand #logo {
+  width: 1.5rem;
+  height: 1.5rem;
+  margin-right: 0.375rem;
+}
+
 /* Override Bootstrap primary with GitHub blue */
 body[data-layout="landing"] {
   --bs-primary: var(--gh-blue);
@@ -44,6 +51,13 @@ body[data-layout="landing"][data-bs-theme="dark"] {
   padding: 3rem 0 1.5rem;
 }
 
+.landing-hero-logo {
+  display: block;
+  width: 7.5rem;
+  height: 7.5rem;
+  margin: 0 auto 1rem;
+}
+
 .landing-hero h1 {
   font-size: 2.6rem;
   font-weight: 800;