docs: add docs for v20 (octokit#340)

----

### Before the change?
<!-- Please describe the current behavior that you are modifying. -->

* Docs would refer to the latest version of Octokit as v19

### After the change?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

* Uses v20 as the latest version

### Pull request checklist
- [x] Tests for the changes have been added (for bug fixes / features)
- [x] Docs have been reviewed and added / updated if needed (for bug
fixes / features)

### Does this introduce a breaking change?
<!-- If this introduces a breaking change make sure to note it here any
what the impact might be -->

Please see our docs on [breaking
changes](https://github.com/octokit/.github/blob/master/community/breaking_changes.md)
to help!

- [ ] Yes
- [x] No

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Author: wolfy1339

docs/gatsby-config.js

@@ -87,7 +87,7 @@ module.exports = {
     {
       resolve: "gatsby-plugin-versioned-docs",
       options: {
-        currentVersion: "v19", // configure the path for the current version
+        currentVersion: "v20", // configure the path for the current version
         versions: [
           {
             name: "v16", // the path of the older version
@@ -104,6 +104,11 @@ module.exports = {
             branch: "18.x", // older versions specify a branch name for this repo
             endpoints: "5.x", // ...and one for the endpoint methods
           },
+          {
+            name: "v19", // the path of the older version
+            branch: "19.x", // older versions specify a branch name for this repo
+            endpoints: "7.x", // ...and one for the endpoint methods
+          },
         ],
       },
     },