Add new explainer frames

Author: DanWahlin

docs/assets/img/ch02-what-is-context.png

@@ -33,7 +33,7 @@
     <h1>Quick Start</h1>
     <p class="b-sub">Install GitHub Copilot CLI (or use Codespaces), sign in, clone the samples, and verify it works.</p>
     <div class="b-track" role="img" aria-label="Course progress: chapter 0 of 8"><i style="width:12%"></i></div>
-    <img class="pipav" src="assets/img/pip-avatar-hat.png?v=8444f7c2" alt="Pip the fox mascot waving"/>
+    <img class="pipav" src="assets/img/pip-avatar-hat.png?v=8444f7c2" alt="Pip the fox mascot"/>
   </div>
 </header>
 <nav class="nav" aria-label="Chapter"><a class="nav-home" href="index.html">Home</a><a class="nav-btn" href="chapter-01.html"><span class="nav-lbl-full">Next Chapter</span><span class="nav-lbl-short">Next</span>&nbsp;&rarr;</a></nav>

docs/assets/img/ch03-ship.png

@@ -33,7 +33,7 @@
     <h1>First Steps</h1>
     <p class="b-sub">See Copilot CLI in action, then learn the three interaction modes.</p>
     <div class="b-track" role="img" aria-label="Course progress: chapter 1 of 8"><i style="width:25%"></i></div>
-    <img class="pipav" src="assets/img/pip-avatar-hat.png?v=8444f7c2" alt="Pip the fox mascot waving"/>
+    <img class="pipav" src="assets/img/pip-avatar-hat.png?v=8444f7c2" alt="Pip the fox mascot"/>
   </div>
 </header>
 <nav class="nav" aria-label="Chapter"><a class="nav-btn" href="chapter-00.html">&larr;&nbsp;<span class="nav-lbl-full">Previous Chapter</span><span class="nav-lbl-short">Prev</span></a><a class="nav-home" href="index.html">Home</a><a class="nav-btn" href="chapter-02.html"><span class="nav-lbl-full">Next Chapter</span><span class="nav-lbl-short">Next</span>&nbsp;&rarr;</a></nav>