Add "noIndex" property to /next/ versioned docs (#54)

The "noIndex" property here adds `<meta name="robots" content="noindex,
nofollow">` to all `/next/` pages, instructing search engines not to
index or follow links on future documentation.

https://docusaurus.io/docs/api/docusaurus-config#noIndex

Note that version options are `current`, `0.11`, `0.12`

* /0.11/ is mapped to `0.11`
* /next/ is mapped to `current`
* / is mapped to `0.12` via lastVersion

Author: Xenc5

docusaurus.config.ts

@@ -86,6 +86,9 @@ const config: Config = {
             [LATEST_DEVVIT_VERSION]: {
               badge: false,
             },
+            "current": {  // This refers to "/docs/next/"
+              noIndex: true,
+            },
             "0.11": {
               noIndex: true,
             },