VitePress: Update logo and favicon (#824)

- Replace old favicon with theme-adaptive (dark/light) versions
- Add website logo
- Standardize homepage image filenames

docs/public/logo-dark.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<svg version="1.1" id="logo"
+  xmlns="http://www.w3.org/2000/svg"
+  xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewBox="0 0 1000 1000" style="enable-background:new 0 0 1000 1000;" xml:space="preserve">
+  <polygon fill="#dfdfd6" points="530,530 900,530 650,650 530,1000 "/>
+  <polygon fill="#dfdfd6" points="470,530 470,900 350,650 0,530 "/>
+  <polygon fill="#dfdfd6" points="530,470 530,100 650,350 1000,470 "/>
+  <polygon fill="#dfdfd6" points="470,470 100,470 350,350 470,0 "/>
+</svg>

docs/public/logo-light.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<svg version="1.1" id="logo"
+  xmlns="http://www.w3.org/2000/svg"
+  xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewBox="0 0 1000 1000" style="enable-background:new 0 0 1000 1000;" xml:space="preserve">
+  <polygon points="530,530 900,530 650,650 530,1000 "/>
+  <polygon points="470,530 470,900 350,650 0,530 "/>
+  <polygon points="530,470 530,100 650,350 1000,470 "/>
+  <polygon points="470,470 100,470 350,350 470,0 "/>
+</svg>