CrateSidebar: Add Package URL help link

Author: Turbo87

app/components/crate-sidebar.hbs

@@ -16,6 +16,16 @@
         <span local-class="purl-text">{{@version.purl}}</span>
         <Tooltip local-class="purl-tooltip"><strong>Package URL:</strong> {{@version.purl}} <small>(click to copy)</small></Tooltip>
       </CopyButton>
+      <a
+        href="https://github.com/package-url/purl-spec"
+        target="_blank"
+        rel="noopener noreferrer"
+        local-class="purl-help-link"
+        aria-label="Learn more"
+      >
+        {{svg-jar "circle-question"}}
+        <Tooltip @text="Learn more about Package URLs" />
+      </a>
     </div>
 
     <time

app/components/crate-sidebar.module.css

@@ -87,6 +87,26 @@
     }
 }
 
+.purl-help-link {
+    color: unset;
+    margin-left: var(--space-2xs);
+    flex-shrink: 0;
+
+    &:hover {
+        color: unset;
+    }
+
+    &:focus {
+        outline: 2px solid var(--yellow500);
+        outline-offset: 1px;
+        border-radius: var(--space-3xs);
+    }
+
+    svg {
+        margin: 0;
+    }
+}
+
 .links {
     > * + * {
         margin-top: var(--space-m);