[website] navigation between doc versions

Summary:
Add a new section in the docs, creatively named "Versions", with links
to the 0.17.0 and "next" version. Staying vague on what "next" points to
because I don't know how often we'll be able to update it yet.

Reviewed By: ngorogiannis

Differential Revision: D21573699

fbshipit-source-id: fc527cf61

website/docs/versions.md

@@ -0,0 +1,8 @@
+---
+id: versions
+title: Documentation Versions
+---
+
+Browse the documentation for different versions:
+- [latest released version (0.17.0)](/docs/getting-started)
+- [next version](/docs/next/getting-started)

website/sidebars.js

@@ -32,5 +32,6 @@ module.exports = {
       "linters-bug-types",
     ],
     Contribute: ["absint-framework", "adding-checkers", "internal-API"],
+    Versions: ["versions"],
   },
 };

website/versioned_docs/version-0.17.0/versions.md

@@ -0,0 +1,8 @@
+---
+id: versions
+title: Documentation Versions
+---
+
+Browse the documentation for different versions:
+- [latest released version (0.17.0)](/docs/getting-started)
+- [next version](/docs/next/getting-started)

website/versioned_sidebars/version-0.17.0-sidebars.json

@@ -117,6 +117,16 @@
           "id": "version-0.17.0/internal-API"
         }
       ]
+    },
+    {
+      "type": "category",
+      "label": "Versions",
+      "items": [
+        {
+          "type": "doc",
+          "id": "version-0.17.0/versions"
+        }
+      ]
     }
   ]
 }