docs(v2): refactor docs for better outline and welcoming PRs (#1641)

* docs(v2): refactor docs for better outline and welcoming PRs

* docs(v2): update docs

website/docs/using-themes.md

@@ -3,8 +3,26 @@ id: using-themes
 title: Using Themes
 ---
 
-TODO: Give a high-level overview about themes and what they can be used for.
+_This section is a work in progress. [Welcoming PRs](https://github.com/facebook/docusaurus/issues/1640)._
 
-#### References
+<!--
 
-- https://v1.vuepress.vuejs.org/theme/
+Outline
+---
+High-level overview about themes:
+- how they are used
+- how to shadow components and the power of it
+
+Related pieces
+---
+- [Advanced Guide – Themes](advanced-themes.md)
+- [API - Themes](api-themes.md)
+
+References
+---
+- [themes RFC](https://github.com/facebook/docusaurus/issues/1438)
+- [how classic template uses themes](/packages/docusaurus/templates/classic/docusaurus.config.js)
+- [using plugins doc](using-plugins.md)
+- [vuepress docs on themes](https://v1.vuepress.vuejs.org/theme/)
+
+-->