feat(guide): add an example of custom component

mock-guide/english/components/example/index.md

@@ -0,0 +1,8 @@
+---
+title: Example component
+component: example/Component/index.js
+---
+## Example component
+
+We can place inline component: <span id='example-component_span'></span>.
+Or place block component: <div id='example-component_div'></div>

mock-guide/english/components/index.md

@@ -0,0 +1,14 @@
+---
+title: Components
+---
+
+## Components
+
+We can use custom components for guide articles. Add
+
+```yml
+component: path/to/component
+```
+
+to an article frontmatter to use your component. Your component should be
+located in `client/src/templates/Guide/components/path/to/component`.