chore/ci: Add DiffTour link to PRs

Adding the shared sourcegraph/actions/diff-tour action to comment on PRs with
the Diff Tour link. Please make any needed improvements to the shared action,
so all of our repos using it benefit.

Author: marcleblanc2

.github/workflows/diff-tour.yml

@@ -0,0 +1,27 @@
+# This GitHub Action imported from the shared action
+# https://github.com/sourcegraph/actions/blob/main/diff-tour/
+# Please make fixes / enhancements in the shared action,
+# so all of our repos get the benefits
+
+name: Diff Tour link
+
+on:
+  pull_request:
+    # `edited` fires when the base branch changes; `synchronize` does not
+    # `closed` fires on merge and on plain close; either way the comment is
+    # updated to a link that survives deleting the head branch
+    types: [closed, edited, opened, reopened, synchronize]
+
+# The only permission needed. The action does not check out your repo, so
+# `contents: read` is not required, even for private repos
+permissions:
+  pull-requests: write
+
+jobs:
+  diff-tour:
+    name: Comment
+    runs-on: ubuntu-latest
+    # Diff Tour resolves branch names against the base repo, so skip fork PRs
+    if: github.event.pull_request.head.repo.full_name == github.repository
+    steps:
+      - uses: sourcegraph/actions/diff-tour@main