[docs] Add links to CNCF information (#2570)

* [docs] restructure

* Add CNCF links

* Update background.md

* Update Footer.js

* Update README.md

microsite/core/Footer.js

@@ -51,6 +51,7 @@ class Footer extends React.Component {
             <a href="https://mailchi.mp/spotify/backstage-community">
               Subscribe to our newsletter
             </a>
+            <a href="https://www.cncf.io/sandbox-projects/">CNCF Sandbox</a>
           </div>
           <div>
             <h5>More</h5>

microsite/sidebars.json

@@ -3,7 +3,6 @@
     "Overview": [
       "overview/what-is-backstage",
       "overview/architecture-overview",
-      "overview/architecture-terminology",
       "overview/roadmap",
       "overview/vision",
       "overview/background",