add howto icon to sidebar & witnessArg img (#384)

website/docs/tech-explanation/witness.md

@@ -33,6 +33,8 @@ table WitnessArgs {
 }
 ```
 
+<img src={"/img/tech_explanation/witnessArg-structure.png"} width={688} height={353} alt="Structure of WitnessArg" />
+
 Under this convention, each item in the witnesses field is a byte array serialized by Molecule from a WitnessArgs structure. Each WitnessArgs contains multiple proof data read by the input Lock Script, input Type Script, and the output Type Script. The order of WitnessArgs item in the `witnesses` field also matters, corresponding to the index of the virtual array created by the [Scripts grouping](/docs/tech-explanation/script-group-exe).
 
 ### CoBuild

website/sidebars.js

@@ -85,7 +85,7 @@ export default {
     {
       type: "category",
       label: "How-Tos",
-      className: "how-tos",
+      className: "category-howto",
       collapsible: false,
       items: ["how-tos/how-to-sign-a-tx"],
     },

website/src/css/customTheme.css

@@ -272,6 +272,12 @@
   width: 1.125rem;
   height: 1.125rem;
 }
+.category-howto > div > a:before {
+  content: url("/svg/icon-sidebar-howto.svg");
+  margin-right: 0.5rem;
+  width: 1.125rem;
+  height: 1.125rem;
+}
 .category-mining > div > a:before {
   content: url("/svg/icon-sidebar-mining.svg");
   margin-right: 0.5rem;