docs: add Diff Tour page

docs/diff-tour/index.mdx

@@ -0,0 +1,20 @@
+# Diff Tour
+
+<Callout type="note">
+	Diff Tour is a beta feature. It is not available on Sourcegraph.com and
+	requires acceptance of the [Sourcegraph AI
+	Terms](https://sourcegraph.com/terms/ai-terms). It does not consume credits
+	during beta.
+</Callout>
+
+Large diffs are hard to review. Unless you are familiar with the codebase, it is often not obvious where to start or which files contain only mechanical changes. Diff Tour turns a diff into a guided reading path. It places important files first, groups related files into sections, and explains the diff, each section, and individual files.
+
+## Open a Diff Tour
+
+Open a diff from the repository Compare page, by comparing branches on the Branches tab, or from a commit page. Select **Tour** next to **Changes**.
+
+For a diff on GitHub, use the [Sourcegraph browser extension](/integration/browser-extension) to open it in Sourcegraph.
+
+## Review a Diff Tour
+
+Read the tour from top to bottom. Open each file to review its changes and mark it as viewed as you go. Sourcegraph generates a tour once and caches the result. Other reviewers opening the same diff receive the cached tour.

src/data/navigation.ts

@@ -105,6 +105,10 @@ export const navigation: NavigationItem[] = [
 					}
 				]
 			},
+			{
+				title: 'Diff Tour',
+				href: '/diff-tour'
+			},
 			{
 				title: 'Code Navigation',
 				href: '/code-navigation',