docs(slides): improve slides images and theme

docs/slides/assets/websocket-history.svg

@@ -17,8 +17,7 @@
   <rect width="960" height="540" fill="#262a3a"/>
   <path d="M34 112 L238 34 L340 146 L156 238 Z" fill="#f4b740" opacity=".12"/>
   <path d="M700 410 L930 318 L982 504 L746 548 Z" fill="#6d8cff" opacity=".16"/>
-  <text x="72" y="76" fill="#f8fafc" font-size="34" font-weight="800">WebSocket History</text>
-  <text x="72" y="112" fill="#a8adbd" font-size="17">From Classic HTTP to Real-time communication</text>
+  <g id="diagram-content-shift" transform="translate(0,-28)">
   <line x1="128" y1="282" x2="832" y2="282" stroke="#4a5168" stroke-width="10" stroke-linecap="round"/>
   <g filter="url(#shadow)">
     <rect x="76" y="164" width="808" height="238" rx="26" fill="#ffffff" opacity=".08" stroke="#ffffff"/>
@@ -60,4 +59,5 @@
       <text x="830" y="390" fill="#a8adbd" font-size="15">Dashboard - IoT</text>
     </g>
   </g>
+  </g>
 </svg>