Switch from Dependabot to Renovate

.github/renovate.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended"
+  ],
+  "semanticCommits": "disabled",
+  "prHourlyLimit": 5,
+  "labels": ["dependencies"],
+  "packageRules": [
+    {
+      "matchPackageNames": ["nbgv", "nerdbank.gitversioning"],
+      "groupName": "nbgv and nerdbank.gitversioning updates"
+    },
+    {
+      "matchPackageNames": ["xunit*"],
+      "groupName": "xunit"
+    },
+    {
+      "matchDatasources": ["dotnet-version", "docker"],
+      "matchDepNames": ["dotnet-sdk", "mcr.microsoft.com/dotnet/sdk"],
+      "groupName": "Dockerfile and global.json updates"
+    }
+  ]
+}

CONTRIBUTING.md

@@ -72,3 +72,13 @@ You can make changes and host the site locally to preview them by switching to t
 After making a change, you can rebuild the docs site while the localhost server is running by running `dotnet docfx` again from a separate terminal.
 
 The `.github/workflows/docs.yml` GitHub Actions workflow publishes the content of these docs to github.io if the workflow itself and [GitHub Pages is enabled for your repository](https://docs.github.com/en/pages/quickstart).
+
+## Updating dependencies
+
+This repo uses Renovate to keep dependencies current.
+Configuration is in the `.github/renovate.json` file.
+[Learn more about configuring Renovate](https://docs.renovatebot.com/configuration-options/).
+
+When changing the renovate.json file, follow [these validation steps](https://docs.renovatebot.com/config-validation/).
+
+If Renovate is not creating pull requests when you expect it to, check that the [Renovate GitHub App](https://github.com/apps/renovate) is configured for your account or repo.